The Eye of Cthulhu is a boss on Terraria. It is the easiest boss in the game. It is summoned by using a Suspicious Looking Eye at night (A Suspicious Looking Eye is made by using 7 Contact Lenses on a Demon Alter). It attacks by dashing toward you or summoning Servants of Cthulhu.
Once you get its health down to 1400 hp or less is changes into something more dangerous like shown below:
*Phase 2 of the Eye of Cthulhu