The Eye of Cthulhu is a bigger version of the Demon Eye and is a boss enemy.
To summon it, you need a Demon Altar and 10 Lenses to craft a Suspicious Looking Eye. The player can use it to summon the Eye of Cthulhu. This boss is thought to be the easiest boss by the games developers.
Using the Suspicious Looking Eye during the day will kill the player instantly. It will also run away if day breaks during the fight.
The Eye of Cthulhu can only be summoned at night and will disappear without dropping any loot if the player does not kill it before day.
Although not required, it's recommended to use a ranged weapon to kill it. Melee weapons damage it, but hitting the Eye of Cthulhu is very difficult, as it sometimes flies through walls and terrain, as well as the air.
The Eye of Cthulhu has 4000 HP.
During Phase 1, the Eye of Cthulhu also periodically spawns Servants of Cthulhu.
Once the Eye reaches half health, the second phase commences. This doubles the amount of damage it deals and changes the sprite so that it looks like a mouth (replacing the iris and pupil) with a set of large, sharp teeth.
In the 2nd phase, instead of spawning servants, the Eyes charge at the player 3 times, takes a short break and then repeats. It continues this pattern until death or daybreak.