This started out being a guide about everything, but then I seperated Tomes from the Achievements to avoid the clutter. I've divided it up into different categories based on the stage.
So Linden Forest, Usvit Depths, Usvit Castle, End of Time and a final section for those things that aren't related to a specific stage.
I've also added some "secrets", but I'm not that sure about if I'm correct about those.


By the way, what are the Tomes of Knowledge? Well from the main menu if you click "Knowledge" you'll see different bonuses that apply to all characters. And at the top there are two tabs.
The rightmost tab opens up the different Tomes of Knowledge which are challenges that reward you with points to spend on said bonuses. The tomes are the unlocked ones and the padlocks are the ones not yet accomplished.

Complete them to get the points, invest the points to get stronger!

An issue I encountered when making the guide is that it seems like a lot of things don't spawn in when you've already cleared them. So I can't really attach a screenshot of everything.

Achievement
This is the first boss of the stage. It usually shows up around the 9 min mark.

Achievement
This is the second boss of the stage. It usually shows up around the 12 min mark.

Achievement
This is the final boss that you fight when you play the stage for the first time around the 19 min mark

Achievement
This is the final boss that you fight when you've kindled the stage, otherwise you'll fight Sporelord Rotwood. (Or does he simply replace Sporelord after you've progressed the main quest?)

I don't know what makes him spawn honestly, but I usually get him when I've kindled the fire. One time before the first boss, one time before the second boss. if you defeat him he will become an ally for you this run. Has a Tome entry, but no achievement.


When playing the stage and after defeating the second boss there can be a notification pointing in the direction of the Armored Snail around the 14 min mark. Go there to defeat it, but it is guarded by spinning shells as well.Not sure if there are any criterias to make it spawn.


This one is a bit odd. When starting a run in the Linden Forest after clearing the stage once there is a chance that a pink pig with a helmet will spawn and then immediately run away. Try to get ahead of it and bump into it to catch it. Dodge rolling seem to help. It picks up speed quickly the more time that passes, so unless you can get it quick it's lost for this time.

Secret boss: Pumpking.
If you play a kindled map and also clear the pumpkin patch event this boss will spawn instead of the second boss (?). Defeat it to unlock the Pumpking Crown blueprint (?).

Events:
Merchant x2
Cursed Altar
Challenge Circle
Pumpkin Patch
Armored Molusk
Invasion: Lord Cerveris

Achievement
This is the first boss of the depths that spawn automatically when enough time has passed (9). This is the fight that transports you to a corridor.


Achievement
This is the second and final boss of the depths that spawn automatically when enough time has passed (18). This is the fight that transports you to a square room.


Achievemet
After completing the Depths for the first time there will be a skeleton ( the dead kind) that appears in the start of the level in sight most likely, bump into it to start this "sidequest".
A window will pop up with two messages on what you need to do to spawn Bilee: one on the right, one on the left. Why not just say what you need to do? Well, because the game randomizes what the tasks are. When you do complete a task you'll see a similar note show up about a medalion part. When you get both you'll hear a roar and a message about "a fearsome echo bellows below the depth ". This means that Bilee will now spawn after defeating the first boss a few minutes later. A dev response mentiones that you have around a 2 minute window after defeating Autlaic to complete the note's task until it's too late for Bilee to trigger.

So what do you need to do?
Splinter ancient clay - Pots
Shatter Sturdy Boxes - Crates
Crush wooden kegs - Barrels
Reduce bones to rubble - Skull piles?
Grave rob the king - Defeat the first boss


In the depths, explore and you'll find sacrophages and skull piles, destroy these to drop a hovering skull pickup, get them all (4?) to unlock the tome and an artifact. The X, which will now be available at the merchant.


Events:
Merchant x2
Cursed Altar
Challenge Circle
Light in the dark

Achievement
Lady Nox is the invader that spawns after the second event but before the first boss around the 4 min mark(?). She will teleport behind you and then charge forwards, or back away and throw daggers at you. What makes this difficult however is the fact that she can stop and heal herself for 200 unless you stun her or deal enough damage. Achievement is for just defeating her.

The rest of the bosses appear in different orders it seems, but around the 9 and 14 mark. But they don't have a related tome.

Achievement
After you have progressed the story by kindling the fire in Linden Forest you can fight the end boss of Usvit Castle He spawns around the 19 min mark. Defeat him and the tome is yours.


In the Castle there are random pages to find. They are spawned at the top or bottom of the closest 5 staircases it seems. So go a little to the right, checking every staircase upwards, then return and check all the staircases downwards and you should have enough. I tried to just keep on going right, but even if they map itself seem to loop a little, there are no pages there, so you must explore around the starting area.


After defeating the second boss of the stage there will be a notification pointing upwards. Go there to find a casket and some elite enemies. Defeat the elite enemies and a vampire will spawn. Defeat it to make it change shape into a bat. Defeat the bat to get the tome and a bluepring for Blood Charm (?).


This refers to having at least 8 Curses active at the same time. Which you will get after continuing after the final boss of a stage and speaking to the NPC by the bonfire. There are two other tomes and some achievements related to surviving curses as well, but they aren't stage specific.


Secret: Harp?
Honestly not sure what this is, it appeared when playing the Castle with the Bogatyr, unkindled just before the final boss.


Events:
Merchant x2
Cursed Altar
Challenge Circle
Whispering Tomb
Vampire Casket
Invasion: Lady Nox

Achievement
This is a kind of event, you'll get a notification to each corner of the stage, go there and deal damage to all four obelisks to get this one. This usually happens around the 4 min mark.

Achievement
This is the double battle of the stage. Defeat them. This usually happens around the 8 min mark.

Achievement
This is the invader of the stage. A ranged enemy, not sure what makes it spawn. But usually spawns around 11 min mark.

Achievement
This is the final boss of the stage, survive and defeat it to get the tome (and lots of souls). Usually spawns around the 14 min mark.

Secret: Fire archer altar
If you kindle the fire then after the first boss you sometime get an event that looks like a cursed altar but it's more orange. Accept the challenge and a ring of burning skeleton archers will spawn to attack you.

Events:
Merchant x2
Cursed altar
Obelisks
Invader: Dancing dragon

Achievement: Activate 3 Cursed Altar Events.
While playing it's common that the game spawns in an event around 3 minutes in. One of the possible events is the Cursed Altar which looks like a hooded statue. Approach it and interact with it to gain a random curse. When 2 minutes have passed, the curse is removed and you get an artifact as a reward for the rest of the run.
For the achievement, this is in total, you don't need 3 in the same run.



While playing it's common that the game spawns in an event around 2 minutes in. One of the possible events is the Challenge Circle which looks like a large circular platform with glowing runes in the middle. Approach it and interact with it to spawn a horde heading for you. When you heave defeated enough of the elite enemies a message will appear that a "cursed challenger has arrived". Defeat it without exiting the circle and you get an artifact as a reward for the rest of the run.


When the merchant event spawns you can interact with the merchant. Two of the item slots on the lower rows are locked when you start the game. Spend the required gold to unlock them for this tome.


Sometimes the Merchant will sell you a Treasure Map for 500 gold, if you buy it you'll get a notification on the screen ( the little icon pointing in a direction) with the image of a chest, repeat this 3 times to get the big reward and the tome.



This is an artifact that you must purchase from the merchant or gain in some other way during your run. The Nightmare Cloak gives you a random curse when equipped but increases your damage by 40%, so deal with it while also finishing the stage.



Like the name suggests. Don't spend your gold on anything. You can still get artifacts from events and other things. But you can't make a purchase. Deal with it while also finishing the stage.


Achievment: Survive 5,10 Curses
This refers to having at least 6 Curses active at the same time. Which you will get after defeating the final boss of a stage and speaking to the NPC by the bonfire. Accept the challenge and you'll get a random curse every 1(?) minute. Each adds another challenge that is likely to get you dead. Stay alive long enough and you'll get this unlock.



Power ups are the bubbly things that gives you a temporary bonus when bumped into. Such as the magnet or the fire breath. You can spend knowledge points to increase power up spawns, but it should come naturally.



You gain abilities when you level up. Some are labeled "active" which are triggered when their cooldown is up, others are passive. You need to get 2 of these active abilities to lvl 5. Remember that you can reroll and banish abilities if you have a hard time getting the right ones.


To get this you need to hit an enemy with a certain artifact equipped.
There is an event in the Usvit Castle (real version) where you get a message about "You hear echoing voices", as far as I can tell this always shows up. This is the event with the massive Tombstone like object that drops souls like crazy. If you break it entirely a one eyed tentacle monster elite enemy will spawn and attack you (no special message when that happens). When you defeat it the first time you unlock a Blueprint for "Cygna's Tentacle" which is needed for this.


This is a Green Item and usually show up in the bottom row in the middle for 500 gold at the merchant once the blueprint is unlocked. So if you've already beaten the event chances are that you already have this available. So buy it in your next run when you see it, you can also reroll the merchant's items.
Now that you have it, keep playing the stage and you'll get the unlock once the artifact is triggered (the tentacle effect hits an enemy)


Unlocking Class Variant: Black Flame Pyromancer
The second time you complete Usvit Castle (by defeating the Ebon King) and you're playing as the Pyromancer, a scroll/tome will be hovering around near where the large soul is, when you grab it you'll get a short message about banishing the darkness in the depths. Grab it before ending your run and you are set for the next part.

The next part is in the Usvit Depths. Like before you need to play as the Pyromancer (?). A few minutes in a notification will show up ( the little icon pointing in a direction) with the image of a black chest. Go there and punch the chest to open it, you'll get a message about finding "and old burned robe". After defeating the first boss you'll get a short message about the robe being infused with darkness.Then simply complete the run of the Usvit Depths and you'll have this variant of the Pyromancer unlocked.

Whenever you play a new stage and choose Pyro, there will now be a choice between Pyromancer or Black Flame Pyromancer. This variant has less health and defense but higher damage.


Play, survive, maybe put ability points into exp gain when leveling up.


Confused by this one? I sure was.
When you start a stage there is usually a blue wizard there standing by a lantern. If you talk to him before maxing your character he'll tell you to get more experience. And that's what you need to purchase perks.
See, perks are temporary bonuses for this specific run alone. Most of these cost around 200-400 exp to gain. Once you buy them, the NPC disappears so you can only make one purchase per run.
Once you've maxed your character's talents this is where you put your exp and eventually you'll get this tome once you've done enough purchases.



There are two steps to this challenge. The first one involving unlocking the Black Flame Pyromancer (See the seperate entry for that).

The next step is to play the three first stages with the Black Flame Pyromancer. Before the first boss or so (depending on the stage), you'll get a notification on the screen ( the little icon pointing in a direction) with the image of a brazier. Go there and stand close to it for the brazier to be lit by the Black Flame.
Repeat this for the other two stages as well and the unlock should trigger when the last one is torched.

Black Flame Pyromancer
Look at the entry above about the Tome for unlocking the Black Flame. Once completed you can choose this version when choosing characters.


Dusk Knight Bogatyr
Simply play the Bogatyr enough to maximize the Talents and it will be available when choosing characters.



Necromancer: New Hollow Flame Evolution for Raise the Dead
Once you defeat Kalinov while playing as the Necromancer, you will unlock hollow flame. At the start of a new run, a mysterious brazier will appear. If you choose to ignite the Hollow Flame, your skeleton summons will evolve at Rank 5 into fire-spitting jack-o-lantern warriors.

This ended up being a bit too much for the guide so I put it into a different guide to make it clearer and easier to follow:

https://steamproxy.net/sharedfiles/filedetails/?id=3382211292