Minecraft wiki entry for The_Wild_Update, associated text: Generates in the deep dark biome.
Made of sculk and a bone-like material.[6]
May be used in redstone devices.[7]
Spreads sculk blocks to wherever a nearby mob dies. The amount it spreads depends on the amount of experience the mob is meant to drop.[8]
This means that even if the mob doesn't drop experience as it wasn't killed by a player, the blocks would still spread based on the amount of experience it normally drops.
Blooms when activated.
Drops only experience when broken without Silk Touch.[4]
Emits light.
Emits soul particles after a mob dies and grows sculk related blocks.