Minecraft wiki entry for The_Wild_Update, associated text: New structure that generates within the deep dark biome.
Includes all of the deep dark features within it.
Made of deepslate and its variants, basalt blocks and its variants, planks, and wool.[27]
Contains soul sand, soul fire and soul lanterns.
Contains candles and skeleton skulls.
Contains chests with unique loot including a unique enchantment, Swift Sneak.[28]
The loot is intended to be items that cannot be found anywhere else in the world, and that provide unique abilities and mechanics to players that weren't possible before and are not attainable by other methods.[29]
Contains mysterious frames made of reinforced deepslate.
Has different levels and open spaces.
Extremely large in terms of size, tends to spawn between y=0 and y=-64.