Bosses

Bosses are large, aggressive, resilient enemies that offer a substantial challenge to players. There are currently 13 bosses, 1 of which is exclusive to the console version of Terraria, and 4 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.

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, 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). Due to how helpful corrupted items are, the EoW will probably be the second boss faced in worlds that have corruption, rather than Crimson.

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. Prior to 1.2, once defeated he could not be summoned again in that world (unlike the other bosses), however as of 1.2 you can summon him again by killing the Clothier with the Clothier Voodoo Doll equipped. 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 20,000 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, unlike the Eye of Cthulhu.

The Destroyer
The hardmode version of the Eater of Worlds, with 60,000 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 25,000 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 58,000 HP. 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.

Brain of Cthulhu
Brain of Cthulhu is summoned with the Bloody Spine. It will also appear every time you shatter three Demon Hearts. It can only be summoned in the Crimson.

Queen Bee
The Queen Bee is summoned by destroying the Queen Bee's offspring encased in honey in the beehive structure, which is located in the jungle, or by the use of an Abeemination anywhere in the jungle.

Plantera
Plantera is summoned by destroying a Plantera Bulb (which spawns after you defeat at least 1 hardmode boss) in the underground jungle after first defeating any other Hardmode boss.

Golem
Golem is summoned by using a Lihzahrd Power Cell at the Lihzahrd altar in the Jungle Temple. In order to access the Jungle Temple, you must first defeat Plantera to obtain the Temple Key.

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.