Mechanical bosses

The term "mechanical bosses" refers to three Hardmode bosses; The Destroyer, The Twins, and Skeletron Prime, which are grouped together under this generic term due to sharing certain characteristics: They can be summoned by "mechanical" items, and are themselves metallic and robotic in appearance, resembling mechanical versions of the pre-Hardmode bosses Eater of Worlds, Eye of Cthulhu, and Skeletron, respectively. In addition, there is a chance that one will randomly spawn each dusk after the first Demon/Crimson Altar is destroyed in a world, and each will continue to spawn randomly until it is defeated at least once. (i.e. chance for one specific boss to spawn when none is defeated, then  chance after defeating one of them, finally  chance after defeating two)

The mechanical bosses' summoning items can be crafted at a Mythril or Orichalcum Anvil. They also drop from all Hardmode enemies (except statue-spawned enemies, Meteor Heads, and Old One's Army enemies) with a chance until their respective boss is defeated.

Defeating any single mechanical boss for the first time will cause the following to happen:
 * The Steampunker will arrive (if a vacant house is available).
 * The Truffle will begin selling the Mushroom Spear and Hammush.
 * Solar Eclipses will now occur naturally.
 * Red Devils and Lava Bats spawn in the Underworld.
 * can be dropped by any enemy in the Jungle biome.
 * Life Fruit will begin to grow in the Underground Jungle (on the versions, Life Fruit begins growing in small numbers when Hardmode is initiated; more will grow after a mechanical boss is defeated).
 * The Traveling Merchant will begin to sell the . On the, he will also begin to sell the Super Star Shooter.
 * upgrades to Tier 2 and will progress to wave 7 (however, Betsy or Etherian Lightning Bugs will not spawn until Golem is defeated).
 * Moss Hornets and Moth will start to drop the Tattered Bee Wing and Butterfly Dust.
 * Plantera's Bulbs begin spawning in the Underground Jungle, allowing Plantera to be summoned.

Defeating all three mechanical bosses causes the following to happen:
 * The status message "" appears.
 * Plantera's Bulbs begin spawning in the Underground Jungle.
 * Reapers and Mothrons now spawn during Solar Eclipses.

Tips

 * In order to acquire the Hardmode ores, it is not required to destroy Altars and thus expose oneself to the risk of having a mechanical boss spawn unexpectedly. Instead, it is possible to open crates, which contain Hardmode ores and bars. As of, crates acquired pre-Hardmode will not give Hardmode rewards.
 * On the, the player can find a Drax in an Underworld Shadow Chest, allowing the player to acquire Turtle armor before defeating any of the mechanical bosses, making it much easier to defeat them on the first try.
 * On the, it is possible to obtain a Terra Blade after defeating a single mechanical boss. Note that three Broken Hero Swords (As opposed to one) from a Solar Eclipse will be required, however.
 * As long as the respective mechanical boss has not been defeated, its summoning item can be farmed in events with a high number of enemies, such as the Pirate Invasion or Goblin Army.

Trivia

 * All of the mechanical bosses shoot a red Death Laser: the Destroyer's segments, Retinazer in its second phase, and the Prime Laser.
 * They are the only bosses whose -exclusive items are crafting materials.