Bosses

Bosses are large, aggressive, resilient enemies that offer a substantial challenge to players. There are 8 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.

King Slime
The King Slime is a giant blue slime with 2000 health. It is summoned with a Slime Crown, and can also rarely appear in the outer thirds of the map completely by itself. As with all monsters that can spawn naturally, the King Slime has a better chance of spawning while holding a Water Candle or under the effects of a Battle Potion. The King Slime is a good challenge for a beginning player, and can be easily defeated by tactical use of terrain, and so, it is not usually considered a real boss.

The King Slime does not provide access to game-advancing items upon defeat, but will drop money and the Ninja clothes.

Eye of Cthulhu
Once the player has 200 HP or more, and has 3 or more NPCs living in houses, and over 10 Defense, the Eye of Cthulhu will have a 1 in 3 chance to spawn at the start of every night, and will continue to do so until defeated for the first time. It can also be summoned using a Suspicious Looking Eye at night.

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. It will also appear every time you shatter three Shadow Orbs. The Eater of Worlds can only be summoned in the Corruption, and if he manages to get out of the corruption, he will flee and disappear. The Eater of Worlds can be a challenge even for intermediate players, as it is a large Worm enemy that travels through blocks, and is made up of multiple sections that can each attack separately (with separate heads, no less) when they are cut off. To defeat this boss, every section must be brought down to less than two body parts to be killed (the smallest possible section is a head and a tail with no body pieces).

The Eater of Worlds is the only source of Shadow Scales in the game, 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 cannot be summoned again in that world (unlike the other bosses). Defeating Skeletron allows you to freely access the Dungeon without being attacked by Dungeon Guardians. The Dungeon contains many unique items, as well as hazards.

Wall of Flesh
The Wall of Flesh is summoned by throwing the Guide Voodoo Doll into a pool of lava in the Underworld while there is a living Guide in the world. Defeating this boss monster will irreversibly enable Hardmode in your world, which will allow the creation of the summoning items for the 3 hardmode bosses. It can drop a variety of powerful items and, with hardmode enabled, players can break Demon Altars in order to spawn the hardmode ores.

The Twins
The Hardmode version of the Eye of Cthulhu. Spawns two eyes with different forms upon reaching 50% health. They have 24,000 HP each. The Twins are summoned by crafting and using a Mechanical Eye at night. It is worth noting that The Twins will gain more defense in their second forms

The Destroyer
The hardmode version of the Eater of Worlds, with 80,000 HP. Although it is similar to its easy mode counterpart, it has some key differences: it won't divide into smaller segments, will shoot lasers from its body if space is available, and will deploy Probes to attack you as well. The Destroyer is summoned by crafting and using a Mechanical Worm anywhere, but only at night.

Skeletron Prime
Skeletron Prime is a far more difficult version of Skeletron, with the head having 30,000 HP. His limbs are different: instead of two hands, he has a vice, a laser gun, a saw and a cannon that shoots bombs. With the head and all limbs combined, Skeletron Prime has 63,000 HP. In addition, unlike Skeletron, Skeletron Prime can be summoned as many times per world as you want, by crafting and using a Mechanical Skull, but can only be summoned at night. It is worth noting that when he headspins his Defense increases, unlike Skeletron, whose Defense decreases when spinning.

Ocram (name subject to change)
Ocram is an upcoming boss for the console version of Terraria. Not much is known about him, other than his shape.

To summon Ocram you must get the "Suspicious Looking Skull" it requires two mechanical eyes, one mechanical skull, twenty adamantite it's bars, ten souls of night, and ten souls of light, craft at mythril anvil.