Dungeon Spirit

A Dungeon Spirit is a small blue enemy that can go through walls. It is the only known source of Ectoplasm.

After defeating Plantera, whenever you kill an enemy with 100 or more health points in the hardmode Dungeon there is a 7.7% chance of a Dungeon Spirit spawning.

They are immune to the Confused debuff.

Buggs
They spawn when Plantera isn't killed yet.