Astlibra is a game full of secrets, and around Chapter 7 you’ll have reached the perfect time to go back for the hidden Legendary bosses sprinkled throughout the game!
There are 6 in total that I’m aware of,that roughly correspond to the Gods in the Mythology book.
They can be done in any order but I’ll be listing the order in what I think is the intended order, given you can access some earlier than others. Granted doing them as soon as you have access is unwise, it’s better to come back when you’re strong and have good skills and Karon Magic. But if you’re up to the challenge go for it.

P.S. If there are any terms or references to Equipment, Items, or Techniques that need some further explanation, it will all be explained in the last section of the guide: 8. Clarification of Items, Skills, Techniques, Magic

First Teleport to Rispadar Castle Town


Enter the transition arch between the item shop and equipment shop, keep going right.


The abandoned house will be on the right of the Hedge arch, enter the house.


There will be a sealed door in front of you, use the Goblin’s Stone Plate item to open it.



Interact with the painting, study it. Once you’re in the level inside, it’s a straight shot towards the boss through a bunch of screens and enemies. Nothing complicated, though there are some large obstacle enemies stopping you from running right through to the boss, so at least kill those. The boss will be at the end!

This one is easy to get to! You may have even gotten here before when you first got to Belkade Volcano
First teleport to Belkade Volcano: Inner Middle Level. Alternatively you can just use the teleportation mirror at your house in Rispadar to spit you out here.


Keep going left, pass the second save crystal you see. Not long now!


Here, go down the first stairway you see, through the screen transition.


Then destroy the rocks suspiciously in the lower center of the new screen you’re on, destroy them and go towards the right, check the entrance way.


And that’s it, you’re in the secret area of Belkade Volcano!

The next secret area and boss is on the Demon King’s Continent.

Some of my recollection of how you access it is spotty as there might have been blockages I destroyed with a bomb or a key I might have used, as I’m reasonably certain you access the area that lets you reach it as part of the main-quest.
It’s possible you’ve already gone to the secret area by mistake and then lamented the fact you can’t teleport out, way back in Chapter 5.

Anyway teleport to Demon King’s Continent: Hut, go left and through the entrance arch.

Keep going left till you come to the hole in the ground two screens away from where you started. Jump in the hole.

You will be in a dark room, what you want to access is to the right, and then down from the save crystal you can see.


Now keep going down.
Note, for some reason I couldn’t actually squeeze my way through the gap to get down here, after I had beaten the secret boss and came back to take screenshots of how to get to the secret area, luckily I still had some screenshots from before I beat it!


You will be treated to a long linear drop, enjoy the scenery, and then get your butt moving towards the right and up to reach the boss of the area. To reiterate, I don’t think I could access the area before the boss after I beat it. So, make sure you have Karon’s nose (Karon Magic) on to see if there’s any chests on the way you might miss. I don’t think there are any, but it helps to be safe.

Now for the secret area and boss you access in Elysium. This is perhaps the single easiest secret area to access in the game.

Teleport to Elysium: Entrance


Then simply keep going left and jump on the hidden platforms (they are shining, peel your eyes for them!) and then access the gateway that will through you into the secret area.

Once inside keep going to the right through multiple screens to get to the secret boss.
I would highly recommend using the Air Surf technique to make getting to the entrance even easier, it’s also very useful for running through all the enemies between you and the boss if you have the Cicada Shell magic equipped as well.

The next secret area isn’t too hard to access, I’m not sure when you first unlock it but what I do know is you can get back into it in Chapter 7.
Given the difficulty of the enemies and HP pool of the boss it feels like the right time to do it.

Anyway, teleport to Rispadar Castle Town, then equip the Pegasus Feather Boots, these will make you immune to terrain damage (i.e. magma) which will be useful later.
You may also want to equip some digging power Karon Magic (Digging Master’s Focus/Finesse), and have two crystal pickaxes handy.

Enter the arch entrance between the item and equipment shop


Keep going right to the hedge entrance, enter it.

Then keep going left till you reach the well. Go down the well. Then keep going left till you reach the entrance to the secret area.


I don't want to post pictures of what the secret area looks like, to keep it a secret! But just go down and to the right on the first screen you get to, do not go to the left since that will bring you to the Rispadar Dungeon you broke out of during the Main Story.

In the secret area there are two digging points that will yield some very high-quality materials. So make good use of your crystal pickaxes. There’s also a lot of lava between you and the boss, so make sure you still have your Pegasus feather boots equipped!

This is it, last secret area and boss I'm going to cover! By the time of writing this guide I haven't actually beaten the boss. After I publish the guide I'll try my hand at it!

This secret area and boss is found is in Belkade Mining Town.

This one requires a bit more technique to access. It’s going to require you to use the Sleeping Medicine you last used in Anesta, and have the ability to Triple Jump and the Jump Boots Karon Magic.
So first equip the Dragon Leather Boots accessory, and equip the Jump Boots and Air Ride Karon Magic.


Keep going right, through both entrances to reach the interior of the Sage’s tower.

When you’re inside, keep going right until you see a ladder peeking out of the top of the screen, triple jump up to it.

Then make your way to the left to access a ladder, funnily enough you only need to double jump to it.

You will be faced with a guard who halts your entrance to the obvious secret area. Just use the Sleeping Aid unique item you made back in Anesta (it's still in your inventory despite being used during the Anesta chapter), while being very close to the guard. This will knock him out. Enter the secret area, destroy the two candlesticks surrounding the Yellow Magic Circle and fight the boss!

Below is a clarification of any specific items you might need, to the best of my knowledge.

Accessories:

Pegasus Feather Boots (Makes you immune to Terrain Damage, all but necessary for the Rispadar Well secret area, obtained as part of the Main Storyline).

Dragon Leather Boots (adds an additional jump, necessary to access the Belkade Mining Town secret boss) Obtained from buying from Mokyuu’s shop, while having the secret formula obtained in the Akrasinon Castle Area ( Purchased from Mokyuu on the Demon King’s Continent, easiest shop access is from the Demon King’s Continent: Port save crystal, his shop is just to the left on the boat. Requires one Chocolate Pie, one Minotaur’s Axe, one High-carbon Steel Ore and around 200 gold. I don’t have the exact number as I have some discount tickets).

Karon Magic:
Exploratory mode:
Karon’s Nose – Two Magic Crystals (adds a counter to show you how many treasure boxes are in an area and how many you haven’t discovered). [Obtained from mastering Ivory Armour, purchased from Mokyuu in the Bridion swamp for one Copper Ingot, two Revolting Meat and one Poinsettia and about 250-300 gold]

Jump Boots – Three Magic Crystals (Enhances jumping power, necessary to access the Belkade Mining Town secret boss) [Obtained from mastering Armor of the Rightful King which you can buy from Mokyuu’s shop on the Demon King’s Continent if you have the formula for it. Requires two Chocolate Cream Puffs, one Heat Exhaust Organ, one Damascus Ingot and about 2200 Gold]

Air Ride – Four Magic Crystals (adds an additional jump, necessary to access the Belkade Mining Town secret boss)

Defensive Mode:
Cicada Shell – Three Magic Crystals (Nullifies the first hit you take, after a screen transition. Forcing another screen transition will reset it. This is really useful for getting through the hordes of enemies that stand between you and the secret bosses, if you just want to run past them all) [Obtained from mastering Shinobi’s Attire, which you can buy from Mokyuu’s shop on the Demon King’s Continent. You can buy it at the same Port shop, for two Damascus Ingots, one Minotaur's Axe and one Heat Exhaust Organ and roughly 2250-2500 gold]

Techniques:

Air Surf – While jumping press a horizontal movement button and the dash button (B in my case) to perform a high speed dash through the air. [Air Surf is a technique found in Elysium, Teleport to Elysium: Inside the Ruins (it’s the room with the dead guards) and then go to the screen on the left, drop down in the hole between the two platforms, going towards the right so you land on the hidden platform, then jump off as much as you can towards the left to access the chest. You may need to activate the chest spawning through hitting a target, or interacting with a secret on the floor. I don’t remember frankly!]