All monsters in Terraria

Boss Monsters are Monsters found rarely in Terraria that are much harder to kill than other monsters. Any boss monster generally has much stronger attacks than a normal enemy, more defense, and considerably higher HP ranging from 1000 to 145, 500. In addition, most boss monsters have their own AI, allowing them set movements to go by. Since update 1.2, all bosses can drop a unique trophy (~10% drop rate) that can be placed on a wall for display.

Certain bosses occasionally summon themselves when the player(or all players on a multiplayer world) meets a certain criteria, differing with each boss. This will be announced with a creepy message at dusk, and about two minutes later the boss will spawn itself, even if the player is not ready, or even AFK. The bosses that spawn themselves in this manner are the Eye of Cthulhu, the Destroyer, Skeletron Prime, and the Twins. King Slime may also spawn itself, but in a slightly different manner. The criteria and message for each boss can be found on their article.

The King Slime, as one might gather, is a slime. It is blue in color and enormous, roughly 10 blocks wide and 5 blocks high. It will decrease in size and spawn blue slimes when it takes damage. It appears to have a Ninja trapped inside it.

King Slime is summoned with the item Slime Crown, crafted with a Gold Crown and 99 gel at a Demon Altar. There has been a bug associated with the King Slime, sometimes causing the boss to not spawn/appear. This may be due to the fact that he spawned inside a cave and cannot get out.

The King Slime actually drops a little amount of Gel when he dies, and all of the blue slimes spawned when the King Slime is attacked also drop gel. The King Slime also drops gold coins, a piece of the Ninja Set and some lesser healing potions.

The King Slime is the biggest slime in the game so far.

The Eye of Cthulhu (commonly referred to as The Eye) is usually the first boss players will encounter. The Eye comes in two forms. The first form has 15 attack and spawns Servants of Cthulhu, while the second form has 23 attack and throws itself at the character``. The Eye of Cthulhu can be summoned by using a Suspicious Looking Eye (which can either be found in random chests or can be crafted from 6 Lenses at a Demon Altar) at night. The Eye can also spawn itself if certain criteria are met. While playing, the player may get the message "You feel an evil presence watching you...", which means that the Eye will awaken soon. After a few minutes the message "The Eye of Cthulhu has awoken!" will appear and the boss will promptly start to attack the player.

Players can summon The Eye anywhere, as long as it is night. Sometimes when underground the player may see "You feel an evil presence watching you...", though the Eye will not spawn naturally whilst they are underground, and will instead spawn as soon as the player returns to the surface.

The Eater of Worlds is a worm-type boss monster. It can be summoned in the Corruption by smashing three shadow orbs or by using Worm Food, which can be crafted at a Demon Altar - however, it can only be summoned in The Corruption.