Bosses

Bosses are large, aggressive, resilient enemies that offer a substantial challenge to players. There are currently 7 Bosses, each with its own particular way of being summoned. Warning messages appear whenever a Boss is about to begin attacking a player. Defeating a Boss concludes with a congratulatory message and several items.

Eye of Cthulhu
The Eye of Cthulhu is summoned with the Suspicious Looking Eye. It also has a chance of appearing on a random night until it has been defeated once; if it is not defeated on its first random spawning, it will have a 1 in 3 chance of spawning each night until it is defeated. It can also be summoned multiple times using the Suspicious Looking Eye, even after being defeated.

Since this Boss can spawn randomly without deliberate action by the player, it is often the first Boss the player must face. It is also considered the easiest Boss to defeat, though it is still a significant challenge, especially to newer players.

Eater of Worlds
The Eater of Worlds is summoned with the Worm Food, or can appear after smashing three Shadow Orbs. This Boss can also be summoned multiple times even after being defeated. The Eater of Worlds can be a tough challenge even for intermediate players, as it is a large Worm enemy that travels through walls, and is made up of multiple sections that each continue attacking when detached from each other.

The Eater of Worlds is the only source of Shadow Scale, and drops a large amount of Demonite Ore.

Skeletron
Skeletron is summoned by speaking to the Old Man at the entrance of the Dungeon at night. This is the only way to summon Skeletron, and once defeated he can not be summoned again. Defeating Skeletron allows you to access the Dungeon without being killed by Dungeon Guardians. The Dungeon contains many unique items, as well as hazards.

Wall of Flesh
Summoned by throwing the Guide Voodoo Doll into lava, defeating this boss monster triggers Hardmode in your world, as well as the following three bosses.

The Twins
The Hard version of the Eye of Cthulhu. Spawns two eyes with different forms upon reaching 50% health. They have 24,000 HP each. Summoned by crafting and using a Mechanical Eye

The Destroyer
The Hard Version of the Eater of Worlds, with 80,000 HP can break apart but not nearly as frequently, when attack in one section multiple times little drones will spawn off of his body. Summoned by crafting and using Mechanical Worm

Skeletron Prime
Skeleton Prime is a far more difficult version of Skeletron,with the head having 30,000 HP, and two additional limbs, adding up to a total of four limbs with a combined health of 33,000. With the head and all four limbs combined, Skeletron Prime has 63,000 HP. In addition, unlike Skeletron, which can only be fought once per world, Skeletron Prime can be summoned as many times per world as you want. Summoned by crafting and using a Mechanical Skull

Minibosses
Minibosses are essentially bosses that do not provide access to game-advancing items upon defeat.

King Slime
The King Slime is a giant blue slime. It has 2000 HP. It is summoned with a Slime Crown, or can rarely appear after traveling far from the center of the map. The King Slime is a good challenge for a beginning player, and can be easily defeated by an intelligent player that uses the terrain to their advantage.

Wyvern
The Wyvern is a flying aerial boss introduced in 1.1. The Wyvern will only appear when players are at high altitudes after having defeated Wall of Flesh. The Wyvern will follow the player down to the ground, where they can use the terrain to their advantage, much like with King Slime. The Wyvern is the only source of Souls of Flight as of 1.1, and it drops anywhere between 5 and 13 of them at a time.