Cavern

Access and exit
The Cavern can be accessed from the Graveyard on all difficulties after defeating the Hand of the King at least once and having first accessed the area with the. Once the key has been used to open the biome, the Cavern remains permanently accessible from the Graveyard, and after the Giant has been defeated once, it can also be accessed from the Slumbering Sanctuary through a 2 BSC door.

The exits from the Cavern include Guardian's Haven and the Mausoleum.

Scrolls
The Cavern contains 6 scrolls, including 4 Scrolls of Power and 2 Dual Scrolls, which cannot spawn in areas requiring the Teleport, Ram or Spider runes. On (4+ BSC) there is a bonus Scroll of Power. When 3 Boss Stem Cells are active, this biome has 4 guaranteed Scroll Fragments, and when 4/5 Boss Stem Cells are active, this biome has 5 guaranteed Scroll Fragments.

Enemy tier and gear level scaling
In the table below, you will find the gear level and enemy tier of the Cavern based on difficulty.

Main level

 * 1 treasure chest
 * 10% chance for a cursed chest
 * 2 Skill shops
 * 1 Weapon shop

Boss Stem Cells rewards

 * 1 BSC: Treasure chest
 * 2 BSC: Treasure chest
 * 3 BSC: Chained items
 * 4 BSC:

Secret areas

 * The blueprint for the can be found in a secret area near the end of the level after a series of traps. In order to reach it, the player must use the  and the.
 * The blueprint for the can be found by unlocking a door with the, which is hidden throughout the level in a secret area on the roof. While it can be found on any difficulty, the door itself can only be accessed on Nightmare/Hell mode, as it is located behind a 4 BSC door.

Enemy blueprints

 * The blueprints for, and  can be looted from Ground Shakers.
 * The blueprints for and  can be looted from Arbiters.
 * The blueprint for can be looted from Skeletons.

Enemies
The Cavern is home to a few unique enemies, including Ground Shakers, Arbiters, Skeletons and Demons, in addition to more common ones. On higher difficulties, Lacerators are replaced by Failed Experiments.

In the table below, you will find which enemies are present in the Cavern depending on difficulty level. For each enemy, any common, uncommon, rare or legendary blueprints they carry are indicated. When applicable, the minimum difficulty level for blueprint acquisition is specified in brackets.

Alchemist grimoires
A research grimoire can be found next to barrels of crystals and a vat containing a body. The Alchemist was apparently trying to use the crystals as the ingredient for a cure for the Malaise:
 * "We can do more with those crystals than sell them, I'm sure of that..."
 * "Using them on corpses doesn't do anything for now, but I have to keep going!"
 * "This Alchemist... Is he a madman or a genius?"

King statues
A statue of the King can be found in the Cavern, next to barrels of crystals. The Beheaded notes how inflated the King's ego appears, that he would go so far as to bring statues all the way to the Cavern:
 * "Seriously? The King left one of his statues here?"
 * "Is there no end to his ego?"

Miners
A lore room full of crystal barrels under the King's banner contains a letter left by a miner. It illustrates how isolated the miners were from the island's population, knowing little of the raging epidemic and the King's terrible measures to stop the spread of the Malaise. Despite being uninformed about the outsides of the Cavern, the miners hid weapons, presumably to defend themselves against the strange creatures within the caverns, and now the monsters from the Malaise:
 * "A letter from a soldier."
 * "There are two kinds of people in the world: those with loaded crossbows and those that dig."
 * "... We dig."
 * "We've dug for 7 months. 7 months here and still no news of the outside."
 * "I hope everything is fine."

A storage room full of crystals and banners of the King contains a letter left by a watchman, which alludes to the Giant. The letter states that the Giant, who previously guarded the Castle gates, has been slain due to an unknown feud between the King and him:
 * "The giant who was watching the castle gates is no more."
 * "What happened between him and the King, I wonder?"
 * "... I don't know what the Giant did, but judging by the spear, they must've had quite the... difference of opinion."

Trivia

 * Assets for the Cavern were already present in the game files in early access, long before the biome was officially added.
 * Occasionally, the Clock Tower can be visible far in the background of the level, beyond the Cavern itself.
 * In a previous update, Cavern used to give the highest gear level in the game since its flawless door to Guardian's Haven would give XIII gear on 4+ BSC, 1 tier higher than its current gear level.