Spookit

Spookits are a unique monster found in forest and midnight biomes. They also appear in the Desert Subsurface underground layer as well as in underground layers 0b, 1d and 5a. As of Version 1.2.0 they can now be found guarding Ancient Essence Chests inside Ancient Vaults with electric and poison themes as well. They float after the player through the air and can phase through blocks. When attacked it closes its eye and becomes transparent and immune to being attacked until it opens it eye again, although they will still be poisoned/on fire. While the eye is shut, the Spookit is unable to damage players. This monster has a resistance (-50% damage taken) to non-elemental damage. When it closes its eyes it becomes immune to attacks by entering a different plane of existence.