Bosses

Bosses are large, aggressive, resilient enemies that offer a substantial challenge to players. There are currently 15 bosses, 1 of which is exclusive to the console version of Terraria, and 6 of which are exclusive to the PC version. 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.

During Hardmode, each night has a one in two chance of spawning one of the three mech bosses that you have not defeated yet.

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. It is also the only boss not to drop a trophy. The King Slime is the most wasted boss as it is spawned by the Slime Crown which needs 30 gold bar and a ruby with the including of 99 gel.

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

Eye of Cthulhu
Once the player has 200 HP or more, 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. It travels through blocks, and spawns Servents of Cthulhu.

Eater of Worlds
The Eater of Worlds will appear every time you shatter three Shadow Orbs. These orbs are found in the Corruption, surrounded by Ebonstone. Since you won't be able to mine Ebonstone until after you've faced the EoW, you must find them first from other caverns and then use bombs/dynamite to reach them (or Purification Powder). Once you are close, they can be destroyed with a hammer. The Eater of Worlds can only be summoned in the Corruption, and if it manages to get out of the corruption, it 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 can also be summoned with the Worm Food.(The crimson equivalent is the Brain of Cthulhu.)

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 and activating his curse at the entrance of the Dungeon at night. It can be summoned again by killing the Clothier with the Clothier Voodoo Doll equipped at night. Defeating Skeletron allows you to freely access the Dungeon without being attacked by the Dungeon Guardian. The Dungeon contains many unique items, as well as hazards. It is not recommended to enter without strong armor.

Wall of Flesh
The Wall of Flesh is summoned by throwing the Guide Voodoo Doll into a pool of lava in the Underworld while the Guide is alive. It is known by most players as the hardest pre-Hardmode boss, at 8,000 health, and defeating it will irreversibly enable Hardmode in that given world. Its defeat will also allow the creation of the summoning items for various Hardmode bosses. It can drop several powerful items, one of them being the guaranteed drop Pwnhammer which can be used to break Demon Altars in order to spawn 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(24,000 each on the Console version). 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 two eyes have different fighting styles. Spazmatism focuses on just hitting you with his body. It also shoots green flames. Once it reaches its second form, it will look like a mechanical version of the Eye of Cthulhu's second form. It will deal a lot more damage, and the flames it shoots are more like the Flamethrower instead of the Cursed Flames, but the flames will still inflict Cursed Inferno debuff on you. The other eye, Retinazer, focuses on shooting you with Death Lasers like the Destroyer's Probes. Once it reaches its second form, it will replace its pupil with a laser cannon. During this form, it will shoot lasers extremely fast, sometimes having the same fire rate as a Megashark. The last eye destroyed will drop all of the loot.

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. The Destroyer will shoot Death Lasers at you, increasing the attack speed with the lower health it has. Weapons that do damage to multiple segments like Flails, Water Bolt, and the Demon Scythe are recommended, because they will do a lot of damage as The Destroyer passes through them. As of 1.2.3, The Destroyer's drops now come from the closest body part.

Skeletron Prime
Skeletron Prime is a far more difficult version of Skeletron, with the head having 25,000 HP(30,000 on the Console version.) 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. However, like Skeletron, you only have to kill the head. 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 is dropped to 0 when spinning. He will drop Hallowed Bars and Souls of Fright, which are both used to craft many kinds of armor and tools. Be warned that when Skeletron Prime is still alive during morning (4:30 AM to 7:30 PM), he will be in Dungeon Guardian mode and effectively one-shot you.

Brain of Cthulhu
The 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. It cannot be harmed at first until you kill all of the Creepers around it, after they are all killed, it will go into its 2nd form, where it will charge at you and has only 1000HP. It is the only boss that is not immune to knockback. It will also teleport while it is chasing you, so you might want to have a ranged weapon.

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's Bulb in the underground jungle after first defeating any other Hardmode boss. As of 1.2.3, you'll first need to kill all three Hardmode bosses. Plantera has two stages: In the first stage Plantera clings to solid blocks using vine-like extensions. In this stage Plantera fires spiky projectiles much like Spiky Balls. In the second stage Plantera has increased speed and charges at the player. Once Plantera is defeated, the dungeon in that world will automatically have hardmode monsters in the dungeon.

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. Golem has two phases. During the first phase, the pieces you can attack are his fists and his head. Once his fists are destroyed he can not shoot them at you, which is recommended before you begin the second phase. After you destroy his head, the second phase begins, and all parts of his body will be vulnerable. The body jumps around a lot more during this phase, and the head will remain in battle, flying above the body shooting fireballs and lasers, but can not be hit.

Mourning Wood
The Mourning Wood is a semi boss that spawns during the Pumpkin Moon. Like many other bosses, it has the ability to move through necessary blocks. It shoots flaming projectiles and has large amounts of hp. It is recommended to fight from a distance.

Pumpking
The Pumpking is found during the Pumpkin Moon event. Multiple Pumpkings can be out at the same time, up to six at the final, fifteenth, wave (or even more in multiplayer). It has three modes of attack, indicated by the face carved in the pumpkin it uses as a head.

Everscream
The Everscream can be found during a Frost Moon and acts much like the Mourning Wood. The biggest difference is that its projectiles can NOT go though blocks

Santa-NK1
The Santa-NK1 is the second boss you encounter in Frost Moon event. Some people consider him harder than Ice Queen. He attacks using his minigun, that cannot fire through blocks and rockets that go through blocks (similar to Mourning Wood). His third attack is a shower of slow falling presents.

Ice Queen
The Ice Queen is the third and final boss you encounter in the Frost Moon event. She has three alternate attacks. She drops the Baby Grinch Mischief's Whistle, Reindeer Bells and also endgame weapon for each class.

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, spear and repeater, Tizona, Tonbogiri and the Vulcan Repeater. It is also considered the hardest boss in the console version.