Minecraft wiki entry for The_Wild_Update, associated text: New biome at the "deepest depths of the world".
Originally planned for 1.17, then moved to 1.18, and was then postponed to 1.19 due to increased scope.
May not contain any water or lava aquifers.[30]
May be the rarest cave biome in the game.[31]
Tends to generate under continental/mountainous areas.[32]
Generates exclusively within the deepslate layer between Y=-1 and Y=-64.[33]
Contains ancient cities.
Wardens can spawn there through the activation of sculk shriekers.
Contains all sculk-related blocks.
With the exception of spawners, no mobs other than wardens can spawn there, similar to mushroom fields.[18]