Dungeon Spirit

A Dungeon Spirit is a small blue enemy that can go through walls. It is the only known source of Ectoplasm. They will not spawn until you have defeated the hardmode Jungle boss, Plantera.

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.

In Multiplayer they sometimes spawn without having killed Plantera, but less frequent.