Sculk Catalyst is a large block crowned with four bony horns, generating naturally throughout the deep dark biome and inside ancient cities. Its function is mechanical rather than decorative: when any mob dies within an eight-block radius, the catalyst absorbs the experience the kill would have produced and instead converts surrounding stone, dirt, and similar terrain into sculk family blocks — sculk, sculk veins, sensors, and shriekers, with proportions roughly tied to the dying mob's XP value.

This means the deep dark biome is not strictly static: bring hostile mobs into a catalyst's range, eliminate them, and the surrounding terrain slowly transforms over the course of seconds. Players exploit this for two purposes — building automatic XP farms (pipe mobs to a kill chamber rigged with catalysts) and creating themed environments by deliberately re-shaping non-deep-dark areas into sculk-covered surfaces. Mining the catalyst itself requires silk touch; without it, the block drops nothing useful, just a small amount of experience.