Main article: Bosses
The Eye of Cthulhu has a 1 in 3 chance of spawning at the beginning of each night once the player has 200 HP or more, 3 or more NPCs living in houses, and over 10 defense. It will continue to do so until defeated for the first time. It has 2,800 health in all (3,640 in Expert Mode). It can also be summoned using a Suspicious Looking Eye at night. This is generally the first boss a player must face. It travels through blocks, and spawns Servants of Cthulhu.
In Expert Mode, the Eye of Cthulhu will do a fast chain dash in its second phase, using the attack more often the lower health it has. This can lead to it dashing almost continually at very low health, making it especially difficult for players in the early game to finish it off.