True Eye of Cthulhu

Not much is known about this creature. It is summoned by the Moon Lord, who is considered to be the 'final boss' of the game.

Trivia
The name of this enemy tells that the Eye of Cthulhu boss fought earlier in the game isn't actually Cthulhu's eye, and judging by the fact that the eye originates from the Moon Lord, it is highly likely that the Moon Lord is in fact Cthulhu, or at least directly related to Cthulhu somehow.