Eye of Cthulhu
This is the first boss in the game. It is a single floating eye that
The enemy will release miniature eyes, called "Servant of Cthulhu". The eye will spawn around six of them before attempting three dives at the player. These servants are easily handled by shoowing at the main iris to take them out as soon as they spawn, but if one slips through, switch to the broadsword.
When it is at half-health, the eye will spin and shed its iris to reveal teeth. It sheds its defense, but makes stronger charging attacks at the player.
It appears randomly at night, when at least one player has 200 Health, and is announced by the message "You feel an evil presence watching you...", or may be summoned manually with the Suspicious Looking Eye (crafted from 6 Lens). It will retreat once daylight approaches.
Eater of Worlds
This boss is only found in the corruption.
It is summoned by destroying three shadow orbs, or by summoning it with Worm Food. To create worm food, you need 30 units of Vile powder (created from vile mushrooms or purchased from the Dryad during Blood Moon), and 15 Rotten Chunks (obtained by killing monsters).
Skeletron
Skeletron is the guardian for the dungeon, and may be summoned only at night by speaking with the old man.
When dawn approaches, it receives a massive attach and defense boost, which will kill any player.
King Slime
This boss has a rare chance of spawning (1 in 300 per slime), but only when away from the initial starting place. A water candle increases the rate at which slimes are created, or summoned by a Slime crown (Requiring 99 Gel, a Ruby and 30 gold bars).
Wall of Flesh
In the underworld, you will encounter a Voodoo Demon, that will drop a voodoo doll. When this dall hits the lava, the Wall of Flesh is spawned.
Defeating this boss will increase the difficulty of the world, by creating new monsters that will cause problems.
The Twins
These two eyes are summoned only at night, using the Mechanical Eye. Constructing one requiers 5 Copper Bars, 5 Iron Bars, 3 Lens, and 7 Souls of Light.
The destroyer
This is an upgraded version of the Eater of Worlds. It is summoned by a Mechanical Worm, which is constructed using 7 Sould of Night, 6 Rotten Chunks, 5 Copper Bars and 5 Iron Bars.
Skeletron Prime
This is an upgraded version of Skeletron, and is summoned using the Mechanical Skull. You will need 5 Copper Bars, 5 Iron Bars, 5 Souls of Light, 5 Souls of Night and 30 Bones.
When dawn arrives, Skeletron will have increased attack and defense, and will kill the player on a single hit.