Phantom Debris

The Phantom Debris is a Hardmode enemy that spawns in the Underground layer and resembles a background rock. It initially moves very quickly, but slows down noticeably the closer it gets to the player. It constantly emits a magenta glow, allowing it to be easily seen in dark caves.

After Moon Lord is defeated, its stats are buffed and it begins dropping Phantoplasm, a versatile crafting material.

Trivia

 * It is one of two enemies which can drop Phantoplasm that spawn in the Cavern layer, the other being the Overloaded Soldier. This is unlike the other phantoplasmic creatures which are found in the Dungeon.