Black Crystal
Black Crystal is a dark ethereal world full of crystal stalactites and stalagmites. Black Crystal doesn't introduce any unique enemies but spawns some of the toughest, Puffer Men, Eye Demons, and Skeletons. The portal out of Black Crystal will appear after defeating the final boss, the Knight of the Crystal. Upon entering this portal, the player will be taken to an amber colored room with green eyes in the walls and pillars, and a large bright yellow crystal in the middle. Interacting with this crystal will give Ending 2: True Death.
Black Crystal starting room
Eye Demon and crystal stalactites, stalagmites
Access
Black Crystal is a secret floor. One community guide says it only becomes available after both Frog World and Blood Rooms have been cleared in the same run, after which a portal can appear in the secret rooms of the floors that follow Blood Rooms. A Steam thread describes it more loosely as a secret portal somewhere on the last regular floor. It is not confirmed whether Blood Rooms must be cleared last.
Defeating the Knight of the Crystal here also earns an achievement, and the crystal leads to Ending 2. See Achievements.
Community-sourced (added October 2026, not yet verified in-game). Sources: Steam guide: "every achievement guide" (MSK); Steam thread: Questions about Endings.