Eye of Cthulhu

The Eye of Cthulhu is a Boss monster that the player can fight. It can be summoned by creating a Suspicious Looking Eye at a Demon Altar. In order to summon the boss, it has to be Night time; otherwise, nothing will happen. It is also possible for it to spawn soon after the message "You feel an evil presence watching you" appears. This happens when the player has over ten hearts and hasn't defeated the Eye of Cthulhu before. The boss has been described as the easiest by the game's developers.

Strategy
Here is a video tutorial on how to defeat the Eye of Cthulhu. Not everything in this tutorial is recommended to have, but it sure does help. http://www.youtube.com/watch?v=VBPNAAVgs-I

The Eye of Cthulhu acts like a larger Demon Eye, flying around and trying to ram the player, often going through walls and underground. Because of its large size, it is recommended that the player brings a bow or Shurikens to the fight, as the player can jump and dodge while constantly hitting the Eye.

A good area to fight the Eye is high in the air on a long platform, so when the Eye flies downwards, you can still attack it with range weapons.

When the Eye starts to spawn Servants of Cthulhu, if the player stands still the Eye will position itself directly overhead. It is possible use this to your advantage by hitting it rapidly with ranged attacks, preferably one that passes through multiple enemies like Shurikens or the Ball 'O Hurt. Not only will this damage the Eye of Cthulhu, but the Servants of Cthulhu that it spawns will be killed instantly.

After the Eye loses 1500 health, it will transform, replacing its pupil with a large mouth. In this form, the Eye of Cthulhu is faster, deals more damage, has lower defense, and no longer spawns Servants of Cthulhu. After it is defeated, the Eye of Cthulhu drops several items, including Demonite Ore.