Introduced in the second content pack, Nightmare King Grimm is a strong contender for the most difficult boss fight in the game. He takes the already fairly challenging Troupe Leader Grimm fight, and turns it up to eleven; attacking faster, dealing two masks of damage with every attack, and leaving fewer openings to lay on the damage.

You will want to be fully prepared before you face down this Lord of Nightmares, so make sure you have all of the following before going in:

• All 16 Mask Shards (for a total of 9 health)
  
• All 9 Vessel Fragments (Slightly less important than the rest of these, since you'll often be spending soul faster than you generate it. Still, it's strictly a positive to have them)
  
• All 8 Extra Charm Notches (11 notches in total)
  
• The Pure Nail
  
• The Shade Cloak
  
• The Monarch Wings
  
• Any Charms you want to use

Got all that? Good, then it's time to begin.

Even moreso than the other bosses in Hollow Knight, Nightmare King Grimm rewards pattern recognition and reaction time. All of his attacks are distinguishable and all of them can be dodged if you react in time.

Note: These are the methods I found most reliable for dodging during my attempts to take down the Nightmare. For a more in-depth look at how to avoid all these moves, check the acknowledgements at the bottom of the guide.

• Dash-Uppercut: If you can get the timing down, dashing through him with the Shade Cloak can work well; if you go too early and he steps back, immediately full hop and either dash towards him again or nail bounce off him. After the uppercut portion, the fireballs are positioned based on where Grimm was, immediately to the sides of where he disappeared should always be safe.
  Probably the most reliable way to dodge this attack is surprisingly simple: just walk away. NKG appears far enough away from you that just turning around and walking the other direction - you don't even need to dash - is enough to avoid both the dash and uppercut, while also leaving you in a very good spot to avoid the fireballs.
  
  
• Fire Bats: Grimm will always fire these shots in the same pattern: high-low-high-low, and the projectiles have slight homing properties. If you approach Grimm before he has fired the second high shot (the third shot overall) he will teleport away and fire both a low and high shot at the same time. You should wait and jump over the first low shot, and from there you have 2 options: if you are far enough back you can heal, as the second low shot should be angled upward slightly and pass over your head, or you can use the Shade Cloak to dash through the second low shot and either Dash Slash, strike, or Shade Soul him. Be aware that each shot is accompanied by a burst of flame above Grimm, so pogoing is not safe.
  
  
• Spikes: You can’t hit him here, so you can take an opportunity to heal if you don’t have Deep Focus equipped. With it the timing may be a little tight and is unsafe.
  
  
• Flame Pillars: This will fast become your favourite move of Grimm’s. With Shape of Unn you can easily get 2-4 heals off depending on your charm loadout as long as you don’t corner yourself (if you have Spore Shroom, try and get him in the cloud asap, as the time he spends hovering lets it do a fairly significant amount of damage over the course of the fight). It’s also your biggest opportunity for damage. The timing is tight, but if you can get under him right as a pillar goes off you can squeeze in an Abyss Shriek. A less risky but more technically demanding option is to jump up and hit him with a full Cyclone Slash, as you can drift in towards Grimm to avoid the pillars, as opposed to being locked in place for Abyss Shriek, with the added bonus of being one of the more reliable ways to generate soul. Even without using those options, this is still the only time in the fight where Grimm just sits there; get as many hits in as possible, just don’t stay in one place for too long and watch out for the damaging areas that remain after the pillars dissipate.
  
  
• Dive Kick: Probably the most annoying move to dodge, due to the damaging trail Grimm leaves. Dodge to the side to avoid the initial dive, then jump and downstrike for a chance at some damage. Whether you connect or not, immediately dash in the opposite direction as Grimm so you don’t run into him or the trail he leaves.
  
  
• Flamespawner: NKG will immediately use this move if you hit him while he bows at the start of the fight, and otherwise when he reaches 75%/50%/25% health, so be ready for it. He is invulnerable to damage from your nail during this attack, but damage from Charms and Spells will go through, so if you are confident in your dodging abilities, feel free to throw a Shade Soul or 2 his way. You should try to avoid staying too close to the wall, as you often need the space behind you to dodge. Don’t jump too high or you won’t land in time to dodge the next shots. If you have to dodge high twice in a row, remember you have a double jump, landing between shots is not a requirement. Don’t forget that you have the Shade Cloak either, sometimes you can save yourself by dashing through a shot instead of dodging it, just take the next shot into account when doing so. If you have the room for it, you can also equip Dashmaster for the ability to dash downwards and easily reset your position between shots. Descending Dark can also be used as an expensive way to either quickly return to the ground or as a last ditch effort to i-frame through a shot.
  Dream Shield, while expensive, will occasionally block the shots, making it a little easier to survive.
  It is also possible to avoid this attack by nail bouncing on top of Grimm for the duration of the attack, though it requires some precise nail bounces up his side. He will also not begin the move if you are where he appears until you are out of the way, so I'd recommend not attempting it unless you have an inordinate amount of trouble dodging the attack normally.

Note these rankings are solely with respect to Nightmare King Grimm, and have no bearing on how the charms perform in the rest of the game.

Because NKG is a dream fight you can freely use the Fragile Charms without repercussions; they do not need to be made Unbreakable to be worth using.

Grimmchild: As of the Lifeblood update, Grimmchild must now be equipped to face off against NKG. Unfortunately, the little guy won't even join you in the fight, meaning you must now face down the hardest boss in the game with a two notch defecit. You will definitely want to make sure you have all 11 notches.

Phenomenal charms, useful in most any build

• Fragile/Unbreakable Strength: As with Nailmaster's Glory, this is a way to capitalize on the limited amount of hits you can land on Grimm. It has the downside of a higher cost, but it's always active and pulls ahead if you can reliably land more than one strike per opening. Keep in mind that unless you pair this with Quick Slash you are able to get comparable, if not greater damage during Grimms fire pillar attack using Cyclone Slash or Abyss Shriek, though both of those options are much riskier than just attacking.
  
  
• Shaman Stone: This is honestly probably the single strongest charm in the entire game, granting access to two of the three hardest hitting attacks (Abyss Shriek with 120 damage and Shade Soul Flukenest at 112, only losing out to a Pure Nail Cyclone Slash for 147). You can pretty safely put this in any build that has room.
  
  
• Grubsong: Let’s face it: if you're using this guide then you are probably getting hit, a lot. Grubsong helps keep you topped up if you get hit multiple times in a row without being able to land a hit to gain soul conventionally. The added synergy with Weaversong just makes this good in even more circumstances.
  
  
• Shape of Unn: A large part of the difficulty of Nightmare King Grimm is the relentless onslaught of attacks dealing 2 masks of damage and leaving almost no time to heal. Shape of Unn remedies the second part of that quite well, turning some of Grimms attacks into an almost guaranteed heal. Pair with Deep Focus and you can spend much of the fight at or near full health.

Great charms, requires synergies or otherwise falls short of S-Tier

• Deep Focus: This only in A-Tier for the synergy it has with Shape of Unn. By itself the extra time makes it next to impossible to heal outside of when you reach certain hp thresholds and Grimm turns into a swarm of bats for a few seconds. With Shape of Unn there are enough openings in his attacks that you can prolong your life significantly.
  
  
• Sharp Shadow: The major boon granted by this charm is the range on the dash; the extra damage is negligible. The boost in dash distance makes it easier to dodge many of Grimm's attacks, and can help close distance to land hits during most of them.
  
  
• Quick Focus: Similar to Shape of Unn, Quick Focus creates openings to heal where there would normally be none. Particularly during the floor spikes.
  
  
• Quickslash: Less useful than some of the other charms here, but the reduced delay in swinging your nail means you can often take two or three swipes at Grimm where normally you'd only get one.
  
  
• Soul Catcher: Getting as much soul as possible from each strike means you will usually have enough available to heal or blast away at Grimm when you need it.
  
  
• Spell Twister: With the limited soul you have from not being able to hit Grimm often you need to make every shot count. Spell Twister makes using spells for offense less punishing, and helps keep your healing reserves topped up when casting.

Provides some benefits, but mostly for specific builds

• Longnail: Being able to land hits from what would be just out of reach can help accelerate the fight. The extra range also makes it slightly safer to pogo Grimm during his diagonal dash.
  
  
• Spore Shroom: This charm shines in one specific circumstance, but it does well enough there that I felt this spot was justified. When Grimm does the flame pillar attack, getting a spore cloud on him for the duration of the attack can help tremendously with whittling down his hp. It equates to roughly 2 strikes even if you have to use the whole attack for healing. While 2 strikes may not seem like much, keep in mind that you will only be getting 1 or 2 hits in on most of his attacks anyway.
  
  
• Nailmaster's Glory: Nail arts are quite good for this fight as much of the time you are only able to get in a hit or two anyway. Decreasing the charge time on them makes it more likely that you can get one off, and makes getting hit and losing a charge less of a pain. There is usually enough time between Grimm's attacks to charge up without it though, and the charge boost isn't big enough to allow multiple nail arts outside of the Flame Pillar attack.
  
  
• Fragile/Unbreakable Heart: Being able to take an extra hit helps immensely when learning the fight, and being recoverable health is just another plus.
  
  
• Lifeblood Core: Having two extra hits can make the difference between life and death, and because they are temporary, you can more freely use your soul for offense until you lose the lifeblood.
  
  
• Dashmaster: The reduced cooldown on the dash means you can more easily recover from mistakes. If you dash towards Grimm and he steps back before he uppercuts, with Dashmaster you can much more easily dodge back away from him. Can also help with Grimm's desperation attack where dashing downwards gives you more control over your positioning for the next wave.
  
  
• Weaversong: While one weaverling was lost to compensate for the numerous buffs bestowed upon Weaversong in Lifeblood, that is a small price to pay. Going from 4 with 2 damage per tick to 3 with 3 damage per tick actually gives a net increase in damage. This along with the improved ai and new synergies with Dashmaster and Grubsong makes this a surprisingly good use of 2 notches.
  
  
• Flukenest: When paired with Shaman Stone, you can get a lot of extra damage during some attacks than with just your nail. When he shoots the fire bats at you, you can regularly get point blank and unload a shotgun blast of grubs in his face. Unfortunately, the notch cost was increased to 3 in Lifeblood in addition to a damage nerf, pushing Flukenest out of the A-tier of charms.

Situationally useful

• Defender's Crest: A lot of Grimm's attacks will put him in the cloud for a small amount of time, so it can add up over the fight. Pair with Spore Shroom for additional damage during the flame pillar attack.
  
  
• Dream Shield: Will occasionally hit Grimm while dodging, but the real value in the charm is defensive: any projectile that comes in contact with the shield will be destroyed. This can generate openings in Grimm's desperation attack and in general make dodging easier. If the shield was more reliable it would be ranked higher.
  
  
• Mark of Pride: With the decrease in available notches as of Lifeblood, Mark of Pride just doesn't give enough of a boost compared to Longnail to justify easily. If the extra range matters that much to you, don't let me stop you, but I would highly recommend using Longnail instead and saving the notch for something else.
  
  
• Stalwart Shell: Many attempts at Nightmare King Grimm have ben ended by taking hit after hit without being able to right yourself; Stalwart Shell gives a small amount of leeway, especially during the bullet hell phases of the fight.
  
  
• Sprintmaster: A little extra mobility lets you dodge a few of Grimm's attacks without having to worry about overshooting your dodge and hitting a different part.
  
  
• Glowing Womb: You probably have better uses for your soul than this, but reliable passive damage is still a boon.
  
  
• Joni's Blessing: Mostly good for learning the fight, but the amount of extra hp given just isn't enough for 4 notches when Grimm is doing 2 damage a hit, especially after its nerf.

Very small amount of effectiveness, not recommended

• Fury of the Fallen: The only reason this isn't in F-Tier is that it might allow you to reach one of Grimm's periodic openings to heal if you are lucky. That being said, 1 hp is probably not where you want to spend your time, so you should aim for less situational charms.
  
  
• Soul Eater: Grants more soul than Soul Catcher, but the additional 2 notches means you can't take as many other charms. As of the Lifeblood update,this now takes up half your available notches, meaning basically the only time you would use this would be in conjunction with Shaman Stone and Spell Twister as a dedicated 'mage build', which is mostly surpassed by Soul Catcher + Grubsong while leaving a free notch. It's really just not worth the cost anymore.
  
  
• Grubberfly's Elegy: While a ranged attack is actually quite useful against Grimm, there are just too many downsides for me to recommend Grubberfly's Elegy. Primarily, getting hit once means you suddenly have a dead charm taking up 3 notches. To get it back working again, you need to heal twice, meaning you probably want to spend a further 3 or 4 notches on Quick/Deep Focus. Unfortunately, the beams fired by the charm don't grant soul, meaning that even with the aforementioned charms you need to get into close range to generate enough to heal, during which you may get hit again. The drawbacks are just too high for the limited benefits gained.
  
  
• Dream Wielder: Grimm only has one attack where he is vulnerable to Dream Nailing (that being the fire bats), and even with Dream Wielder the timing to do so is incredibly tight.
  
  
• Hiveblood: For 4 notches a charm had better be game changing, and Hiveblood just isn't; if you have no other healing, then it will only grant you an additional 2 hits assuming you can avoid damage until it goes off, though if you are able to get healing in then it can extend your life a decent amount. If you are consistently able to avoid damage for long enough for Hiveblood to be relevant, you are probably better off with a different charm.
  
  
• Kingsoul: Can fit in a very niche build with Glowing Womb and other passive damage sources while you focus solely on dodging, but even then 2 soul per second is not worth more than half of your available notches.
  
  
• Lifeblood Heart: Almost objectively worse than Fragile Heart for this fight. Grants the same amount of health for the same notch cost, but you can never heal it back no matter what. Has a niche use of protecting Grubberfly's Elegy from one hit, but still not recommended.
  
  
• Thorns of Agony: Most of Grimm's attacks are ranged, and those that aren't are fast enough that you probably won't hit him with the thorns reliably.
  
  
• Steady Body: The only real purpose for Steady Body in the fight is to avoid being pushed back into a flame pillar when striking Grimm.
  
  
• Baldur Shell: Can be useful if you know you can't dodge an attack in time. Though keep in mind that it will break after blocking 4 hits, so use it sparingly.

Charms with no effect

• Wayward Compass: If you can't find your way to the fight then you may have bigger problems than this guide can help with.
  
• Gathering Swarm: Gather up all that non-existent geo.
  
• Fragile/Unbreakable Greed: At least it won't break if you die I guess.
  
• Heavy Blow: This isn't even worth it against enemies you can knock back.
  
• Carefree Melody: If you’re facing NKG, then there is no way to have this charm.

Shaman Stone(3) + Spell Twister(2) + Soul Catcher(2) + Grubsong(1)
Notch Cost: 8

This combination of charms is usable practically anywhere in the game, and allows you to output a truly ridiculous amount of damage relatively quickly.

Shape of Unn(2) + Deep Focus(4) + Quick Focus(3)
Notch Cost: 9

Quick Focus is helpful to bring you back from dangerously low health during extended healing opportunities, but the 3 notches can usually be saved as Shape of Unn by itself creates plenty of openings to heal. If dropping Quick Focus from the build, Grubsong can be used to help heal through multiple hits, and Spore Shroom can be used to deal some chip damage when you are too busy healing yourself.

Fragile Strength(3) + Quick Slash(3) + Longnail(2) + Steady Body(1)
Notch Cost: 9

Steady Body is mostly for safety during the Flame Pillars, and can be easily dropped to make room for other charms.

If you remove Steady Body from the build, you can replace Longnail with Mark of Pride for what is potentially more safety than Steady Body provides.

Fragile Heart(2) + Joni's Blessing(4) + Lifeblood Core(3)
Notch Cost: 9

Though this build has been heavily nerfed by the Lifeblood update, it is still decent for learning the fight, as you can take a large number of hits before going down (you will be killed by the 10th hit, down from the 12th pre-Lifeblood thanks to the nerf to Joni's Blessing and the loss of notches for Lifeblood Heart). Once you can reliably dodge at least most of Grimm's moves you should switch to a different build though.

Nailmaster's Glory(1) + Various Defensive Charms(?)

Because most of the nail based charms do not affect nail arts, this is potentially the most versatile build, stack up on defensive charms since most of your damage will be done through Dash Slashes and Cyclone Slashes.

Works well combined with the Self Healing build

Defender's Crest(1) + Glowing Womb(2) + Weaversong(2) + Spore Shroom(1) + Grubsong(1)
Notch Cost: 7

With a heavy focus on damaging NKG without active input on your part, this build can let you focus on dodging, only attacking when you feel like it's safe. Weaversong and Grubsong together help to offset the drain from Glowing Womb when you are unable to gain soul via striking, and make it more likely that you have soul available when you get an opportunity to heal.

Shape of Unn(2) + Deep Focus(4) + Spore Shroom(1)/Weaversong(2) + Grubsong(1)
Notch Cost: 8/9

The first 2 here are fantastic at keeping you alive for as long as possible, with Grubsong comboing particularly well with Weaversong to keep the Soul flowing. In either case, this allows you to sustain yourself while still dealing a decent amount of passive damage to Grimm.

With the advent of Godmaster, the Nightmare King has returned to haunt us once (or twice) more. While the title of 'Hardest Boss' has been passed on, NKG is still no pushover, and is more difficult than ever due to being part of a boss rush/Ascended/Radiant difficulty.

In both of the rematches with NKG Grimmchild is no longer required. He still won't show up either, so you really shouldn't ever use it.

Being killed in one hit from any boss makes them significantly more difficult, and one who is as fast-paced as Grimm becomes an absolute nightmare to deal with. Fortunately, this instadeath isn't all bad: if you die in one hit anyway, overcharming has no downside. Combined with Grimmchild no longer being needed, this means you potentially have a full 14 notches to play around with (technically up to 15, but for the sake of the majority I am going to assume you have already traded the Kingsoul for the Void Heart, and two of the three 4 notch charms affect your health and as such do nothing for us here; Soul Eater can be useful though) make sure you slot the most expensive charm into a single remaining notch for maximum effectiveness. Here is a list of all the charms this affects noticeably:

Health Increasing

• Fragile/Unbreakable Heart
  
• Lifeblood Heart
  
• Lifeblood Core
  
• Joni's Blessing

Only Work After Being Hit

• Grubsong (rip)
  
• Stalwart Shell
  
• Fury of the Fallen
  
• Hiveblood
  
• Thorns of Agony: Can still be triggered with Baldur's Shell, but using notches for a chance at 8 hits worth of damage that require soul for focus and careful tracking of charges left on the shield is not the most useful.

Focus Enhancement

• Shape of Unn
  
• Quick Focus
  
• Deep Focus: While it can still be used to increase the radius of Spore Shroom's cloud, this is not a good use of 6 notches.
  
• Spore Shroom: You can still focus when at full health, but the return is much lower than using the soul for a spell

• Grubberfly's Elegy: The extra notches give you a lot of extra room for the expensive charms, and since you're either at full health or dead, Grubberfly's Elegy is an always on ranged attack, and since you no longer need to heal the beams not granting soul is less of an issue.
  
• Baldur's Shell: When you die in one hit, having a shield to block a few hits you aren't confident in your ability to avoid can be the difference between a failed attempt and a success.
  
• Carefree Melody: Now that you can actually have this charm for the fight, it can really shine. the increasing chance to block damage remains between fights, so on average you should be able to survive a hit once every 4 or 5 attempts; it's not a high chance (at first), but any potential damage mitigation can have a massive impact.
  
• Soul Eater: Normally held back by it's high notch cost, Soul Eater is one of the best charms to slot into the 11th notch for overcharming
  
• Kingsoul: If you happen to still have the Kingsoul, it's now a passable option for overcharming, meaning you can play a little safer and still generate soul without the need to hit Grimm. It's one essentially free spell every 17 seconds (12 with Spell Twister) on top of whatever else you can do. For comparison, Soul Catcher generates enough extra soul for 1 spell over 11 hits (8 with Spell Twister), Soul Eater does the same over a little more than 4 (3 with Spell Twister), and both together takes 3(2 and a bit).

Shaman Stone(3) + Spell Twister(2) + Soul Catcher(2) + Flukenest(3) + Soul Eater(4)
Notch Cost: 14
The combination of Spell Twister + Soul Catcher + Soul Eater makes it so you can cast a spell after almost every attack (22 soul per hit, 24 soul to cast). Flukenest can be dropped if you want the range granted by Shade Soul.

Fragile/Unbreakable Strength(3) + Quick Slash(3) + Mark of Pride(3) + (2 of)Dashmaster(2)/Sharp Shadow(2)/Longnail(2)/Sprintmaster(1)
Notch Cost: 12/13
This setup is made so that you can quickly approach or retreat while dealing as much damage as possible with little regard for soul. Dashmaster + Sprintmaster offers the most general mobility, with the speed increased further by the synergy between them. Dashmaster + Sharp Shadow can increase your damage output if you can consistently nail the timing to Shade Cloak through the initial dash of NKG's uppercut attack.

Fragile/Unbreakable Strength(3) + Quick Slash(3) + Grubberfly's Elegy(3) + Steady Body(1) + Kingsoul/Soul Eater/Mark of Pride(5/4/3)
Notch Cost: 13/14
This build lets you land hits with a consistency unmatched by any other build. The last charm is a matter of taste; Soul Eater adds spells into the mix, with each actual hit being more meaningful, while Mark of Pride allows more actual nail hits overall. If you want to focus more on Grubberfly's Elegy rather than it being an extra source of damage then I'd recommend Mark of Pride for a larger damage increase overall since more nail hits will also grant more soul in addition to the extra damage. Alternatively, Kingsoul can allow you to just stay as far from Grimm as you can and primarily use Grubberfly's Elegy and fire off a Shade Soul every so often.

Fragile/Unbreakable Strength(3) + Weaversong(2) + Defender's Crest(1) + Glowing Womb(2) + Longnail(2) + Soul Eater(4))
Notch Cost: 14
This is the build I used when I beat Radiant NKG. Focusing more on avoidance meant there were less opportunities for spellcasting so I took some passive damage sources and then crammed Soul Eater into the last notch to keep Glowing Womb active and so that I would have some soul when the opportunity for a Shade Soul presented itself.

All images taken from the Hollow Knight Wiki

Thanks to:

• AbyssalAlchemist for a comprehensive list of all the changes in the Lifeblood update
  
• Kanra for this guide to avoiding Grimms many attacks https://steamproxy.net/sharedfiles/filedetails/?id=1345604152
  and also for confirming the Grimmchild requirement

Any feedback is welcome. Thanks for reading!