Want to summon this boss without that pesky 'Suspicious Looking Eye' item? Follow this guide, and you'll have a 33% chance to spawn him every night!
The first requirement is at least 200+ health. Also, I hope you haven't been neglecting your NPCs, because you'll need at least four. You can get NPCs by building houses for them.
A common mistake is trying to summon the Eye of Cthulhu in a world where he has already been defeated. If you joined a game that isn't your own, be sure to ask the creator if the Eye has already been defeated before going through the trouble.
Got enough health and NPCS? Great! Any moment, you should receive a message that says “You feel an evil presence watching you…”. This means that the Eye will awaken the next time the sun goes down. Once you get the “Cthulhu has awoken!” alert, watch out! The Eye of Cthulhu will immediately attack you!
Not quite ready to fight The Eye of Cthulhu just yet? That’s perfectly fine, once you receive the initial “You feel an evil presence watching you” message, either exit the world or the game. This will restart his spawn and you'll get another chance the next night.