Blood Rooms
The Blood Rooms mimic the wall and floor textures of Slime Pits and River, but every texture is blood red (including slime bulbs). This level introduces blood variants of normal enemies, the Blood Skeleton, Blood Spider, and the Blood Slime. These enemies are notably stronger in both health and damage, and all enemies leave behind Trails of blood goo that are not affected by the slime ring ability that negates goo trail damage. The portal back to one of 3 default floors will appear after defeating the final boss, the Blood Golem.
Blood Rooms starting room
Blood Spider and Blood Skeleton
Access
Blood Rooms is entered through a portal that can randomly appear in Secret rooms. Clearing it is needed for the K note, and (together with Frog World) for unlocking Black Crystal.
Community-sourced (added October 2026, not yet verified in-game). Sources: Steam guide: "every achievement guide" (MSK).