Minecraft wiki entry for Sculk_Catalyst, associated text: If a mob dies within 8 blocks of a sculk catalyst, it blooms, giving off soul particles and creates a patch of sculk with sculk veins around that location. Blooming sculk can replace blocks in the sculk_replaceable tag, which includes the surface materials of most of the biomes in all three dimensions. If a mob is killed and detected by the sculk catalyst while over a sculk block, it may occasionally result in the growth of a sculk sensor.