Bosses

Bosses are large, aggressive, resilient enemies that offer a substantial challenge to players. There are currently 11 bosses, 1 of which is exclusive to the console version of Terraria, and three of which are exclusive to the PC version of Terraria. Each has 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.

Queen Bee
Queen Bee is summoned in the jungle biome. The Queen Bee drops the beekeeper, beenades, bee gun, and other bee related items. You must find a honeycomb in a beehive (located in jungle) and destroy the honeycomb. Queen Bee will not be too hard to defeat, but you might need a bit of equipment before trying.

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. gfuphag; aergh

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, although some people think the Eater of Worlds is easier. The Eye of Cthulhu 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 tiles, 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.

Brain of Cthulhu
Brain of Cthulhu is summoned with the Bloody Spine. It will also appear every time you shatter three Demon Hearts. The Brain of Cthulhu can only be summoned in the Crimson. The Brain of Cthulhu has two separate phases during his battle. In the first phase, the Brain itself will be invincible and teleport around the room, and will be surrounded by brain cells that fly at the player, dealing damage. Each brain cell has its own health bar, and will drop Tissue Samples and health pickups. When all the brain cells are destroyed, the Brain can be attacked directly. In this second phase, the brain will continue to teleport, but will concentrate on ramming the player.

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 4 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 his head spins, his Defense increases, unlike Skeletron, whose Defense decreases when spinning.

Golem
Golem is a new boss introduced in the 1.2 update.

Plantera
Plantera is summoned in the jungle biome. Plantera spawns when you reach hardmode, and to spawn him, you must find a pink flower in the jungle.

Ocram
Ocram is a boss that exists only on the console version of Terraria. To summon Ocram you must get the Suspicious Looking Skull. It requires two Mechanical Eyes, one Mechanical Skull, twenty Adamantite Bars, ten Souls of Night, and ten Souls of Light, crafted at a Mythril Anvil. Upon death, he drops 5-10 Souls of Blight. Souls of Blight are used in the crafting of the new console-exclusive armor, Dragon Armor, Titan Armor, and Spectral Armor. They are also used to craft the new sword and spear, the Tizona and Tonbogiri. It is also considered the hardest boss in the console version.