After completing several runs, Esper will have a dialogue prompt and will tell you about the Compendium, which is a giant book that details the monsters, skills, relics, etc. that you have encountered or defeated.

It goes without saying that from this point on, all content is considered spoiler material. I will mark any information that is not strictly need-to-know under the spoiler tag.

The compendium, located at the bottom-right corner of the Nexus.

At the top, there are headers/tabs that you can navigate through using Q/E or the shoulder buttons on your controller. Going all the way to the right beyond the last tab will unlock a secret tab for Secrets. You can also go left from the first tab. Discovering this tab is the first secret. There are currently a total of 12 known secrets for the game.

The secrets tab (the egg icon)

In the main menu, selecting the credits will take you to a minigame in which you are assigned a random weapon and skills and can kill helpers, which respawn around the map as you kill them.

Increase your score by killing helpers. After the timer ends, a root with blue lights will appear in the top-left. Interacting with it will restart the minigame and reset your loadout.

Meeting a minimum score threshold will unlock secret #2 as well as a new legendary relic, Doom Shell.

Doom Shell - "The first time you receive damage in a room, release the Doom Shell." This skill has been updated but I don't have a screenshot. Basically when you roll, you get a doom shell on you that does damage.

I don't know what the minimum required score is, but 30 kills did it for me. I believe this may be the minimum based on the color coding of your score, but let me know in the comments if you managed to unlock it with a lower score.

Apples will occasionally drop from enemies or lootable corpses and will heal you for 5 HP upon consumption.


Heals for 5 HP

Applying fire to an apple through a fire skill (like Ember Fire) or other ways such as spark mod for the ember blade will turn the apple into an apple pie which will instead heal you for 10 HP. A fire wisp expiring on top of it will also count.

Additionally, applying electricity, such as by attacking it with the staff, will turn it into a poutine (?) that gives you one skill charge on your first skill.



Smacking the apple with a hammer will turn it into a brownie that increases your max hp by 1.

Thanks Toadhunter, for the tips on additional methods and outcomes!


Heals for 10 HP

You will also receive 10 embers as a reward.

Attacking an apple pie on the ground will split it into 4 slices of pie, each healing for 3 HP, increasing the total healing by 2 HP, from 10 HP to 12 HP.


Magic!

Using a fire skill on a piece of meat will change it into grilled meat, increasing the healing from 10 HP to 20 HP. The fire skill can be used directly on the object containing the meat; it is unnecessary to break the object to free the meat before using the fire skill. Similarly to the apple pie, if your melee attacks are imbued with burn, then you can cook the meat with just a simple melee attack.


More magic!

Using a fire skill again on a piece of already grilled meat will burn the grilled meat to ashes and reduce the healing from 20 HP to only 5 HP.


Meat's back on the m-- Oh...

In the Nexus, hitting Esper 3 times will unlock Secret #7 and prompt Esper to ask, "Don't you grow tired of this?"

Nope.

Attack the sign at the back of Arlan's shop until it breaks. This will prompt Arlan to express his displeasure but more importantly, it will drop gold and unlock Secret #8.


Still have not grown tired of this

Eventually, the sign will be upgraded from wood to metal and require more hits to destroy. It's okay, though, we've got the time.

Arlan's shop will randomly have a glass vial containing some green substance for sale for 1 gold. Upon purchasing the item, it will drop onto the floor and shatter, releasing a green puddle. The puddle will form a slime and crawl out of the shop.


I'm never gonna financially recover from this.
Image Credit - Bald E on Youtube

In subsequent runs, entering a ??? room may lead you to the "shhhop?" in which there are no enemies, but there is a pool containing a blob creature named "Bob." Interacting with Bob will prompt an offering of gold. I'm not sure how much is required, but he will give you a normal relic called Glass Jar. Festive Feline got it with an offer of 100 gold, which is the lowest reported so far. Secret #12 is also related to this shop.

Support small businesses.

Once you return to the Nexus, Esper will have a dialogue prompt after the run in which he'll inspect the relic and suggest you talk to Arlan about it. When you speak to Arlan, he will repair the jar, unlocking legendary relic Bob's Containment Field and completing Secret #9.


Your first skill use in each room grants a shield for 5s that stops projectiles and prevents damage to all Knights inside it.

Secret #10 is unlocked in the Mimic King room, which was added in the July 27th update (v0.4.1). It can be reached from other worlds but I had the highest chance of encounters in World 1.


Mimic King rooms are marked by a crown in the doorway leading to it.

In the room, you'll be presented with a Three Card Monte-style minigame. Choosing the right chest will reward you with gold and choosing incorrectly will force you to fight mimics.*

Next to the Mimic King is a record player. Hitting the record player will cause the background music to skip and hitting it repeatedly until it breaks will unlock Secret #10. It takes a lot of hits; somewhere around 50.

And if you hit this table one more time then the record might skip-- might skip. I told you... stop hittin' the table.

Thank you to Darogn15 and Nugfrey for providing this info!

* Side note: If you have won the normal three card monte, it seems your next encounters will be a "double-or-nothing" version in which the shuffling is much more difficult to follow. FWIW, I picked right twice and lose both times. I picked left once so far and won.

After winning the double-or-nothing, my next encounter had 4 chests instead, but the shuffling wasn't as difficult. Lower-right was the correct choice for me.

After that, it was 5 chests. Lower-left was the correct choice.

**Note: The 0.71 hotfix changed this secret, but I'm not sure how. Please let me know in the comments if the info below is now incorrect.

In World 1, you may see these waterfalls with a glowing symbol of the Ember Tree on the cliff face.

You have no idea how long it took me to get this waterfall and have the Blast Bomb skill.

Using the Blast Bomb skill on this wall destroy the cliff face and reveal a cave. Upon entering the cave, you will unlock the Ember Pit and Secret #11.


Picking it up will unlock Secret #11.

Back in the Nexus, you'll find the Ember Pit located north of the Compendium. Interacting with it will cycle through the available colors (blue, orange, purple, green)

Fun fact, these are more or less the actual flame colors produced when these chemicals are set on fire. The fire adds energy to the atoms, which cause its electrons to jump to a higher energy state. It subsequently comes back down, releasing the gained energy in the process. Depending on the chemical, this energy is released at different wavelengths, which correspond to varying colors of light. This information is used to help determine the composition of distant stars and is also used to color fireworks.

Thanks, Arcane, for the tip on World 2!

In World 2, there is a similar-looking wall with the same symbol in the room with statues along the left side and ruins on the right. Using a fire skill on this wall will grant you access. Within this room is another ember dust, which gives you another color for your steward.



Thank you do Fenekku Kitsune, Sir Patrick Swayze, and Moo for the following info:
In World 3, a similar door exists and can be lit with a wisp. It rewards you with a pink ember dust.
A wisp gravestone seems to spawn on the room with this door, so taking summon wisp may not be necessary. Please let me know in the comments if you experience otherwise.

After encountering the Mimic King several times (6th encounter for me), you will go into the room and find a sleeping Mimic King. Please note: you will only be able to play the chest shuffling game OR wake him up. Playing the game will result in waking up the Mimic King and you won't unlock Secret #13. Which is what I did. However, the next time I encountered him, he was, again, sleeping.

He's so cute when he sleeps.

Hitting him and waking him up will unlock Secret #12 and also place you into a mini-game of sorts in which chests/mimics will constantly spawn. When you open them, they will either be chests and drop gold/items/heals/relics (yes, even relics), or be mimics. You must defeat a certain number of mimics within 25 seconds to complete the mini-game.

NOTE: If you are lucky enough to drop a relic, you must pick up the relic yourself before you kill the last mimic needed. Otherwise you will be automatically transported back to the Mimic King room without the relic. The same is true of healing items.

Upon completion, you will unlock the Mimic King's Consort relic:

I'm still trying to figure out how exactly this relic works and when the mimic will create stat gems. The mimic can be defeated so you might want to play in such a way as to protect it from getting mobbed. However, it will respawn in the next room.

Thanks, LimeDylan, for providing this info!

At the rift portal after the World 2 boss, this little guy is hiding in the upper-right corner. Hitting him with your weapon will unlock Secret #14 and provide you with a seasonal ember dust, a new color for the steward.

I'm sorry, little snowman...


Lookin' good, though

You're supposed to "help a steward reach its full potential." Walk up to any steward in the base and interact with/talk to it a few times (default RB on xbox controller). Each time you interact/talk, it will puff the steward up slightly until it pops.

Choose your victim student

That's all I've got. Thanks for reading and I hope it was helpful. If any further details are needed, if any of the steps are unclear, or if any information is incomplete or inaccurate, leave a comment and I will try to fix that.