Eye of Cthulhu
The Eye of Cthulhu can spawn with a 1:3 chance each night provided the player meets a few requirements:
• Eye of Cthulhu has not been defeated yet in the current world
• One player has 200 or more HP (10 hearts or more) and also having 10 or more defense
• Three NPCs in your world that are allied with you
If these conditions are met, and the 1:3 chance is triggered, you will see a message "You feel an evil presence watching you." Shortly after this message appears, you will be attacked by it. However, you can force him to spawn by using a Suspicious Looking Eye
IMPORTANT! The Eye of Cthulhu must be defeated before dawn or it will escape just like the normal Demon Eyes when dawn breaks.
The Eye of Cthulhu is a gigantic variant of the Demon Eyes enemies you regularly encounter at night. It is recommended to fight it with a ranged weapon like a Gold / Platinum bow. The Eye of Cthulhu has two forms.
His first form, consists of him hovering around the player and relying on his summons to fight you. His summons are relatively easy to kill but go through walls so focus on keeping their number manageable and keep pounding on the Eye of Cthulhu itself.
During his summons, it will sometimes roar. If it does this, it will most likely do a charge for the player. Dodge it and it will resume to summoning the mini eyes. Just focus on keeping your distance.
After he gets below half of his HP, he will spin around and show his 2nd form. This time, the iris will rip open to reveal a mouth with sharp teeth. In this mode, he forgoes playing defensively and goes on the aggressive.
It no longer summons mini-eyes but charges furiously at the player. It charges 2-3 times after which, you may freely attack it. It gets a lot more aggressive and does more damage but it also takes a lot more damage at this form so just keep running and firing backwards.
Upon reaching the end of the Arena, use the platforms to maneuver vertically and keep running while firing. Keep this up and the Eye of Cthulhu will go down in no time.