eye of cthulhu

One of the first bosses in Terraria. Named after Cthulhu, this disembodied eye, when reaching around half health, turns into a mouth. It has a chance to spawn naturally at night once a player has reached 200 HP or more.

In Hardmode, there is an upgraded version of this boss known as the Twins, consisting of Retinazer and Spazmatism.