The Eye of Cthulhu is a boss mob from the Terraria Idea Pack. It is summoned by the Eye of Cthulhu Summoner,
The Eye in both its phases as it appears in Terraria.
which is crafted with a Fake Eye and a Gold Ingot in a shapeless recipe. Eyes of Cthulhu can also spawn with a 1/20 chance at night when the player has obtained at least one Diamond.
Fight
The Eye of Cthulhu flies about the land, occasionally plunging down to attack the player. Every game tick, it has a 1/20 chance to spawn a Servant of Cthulhu, which attack similarly to it. When the Eye is lowered down to half health, its iris splits open and reveals a mouth with fangs. It does more damage and is faster in this stage, but the Servants stop spawning. If hit outside the mouth, it will only take 1 damage.