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, Skeletron, and Eye of Cthulhu. In addition, there is a 10% chance that one will randomly spawn each night 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.

The mechanical bosses' summoning items can be crafted at a Hardmode Anvil. They also drop from all Hardmode enemies (except statue-spawned enemies and Meteor Heads) with a 1/2500 chance until their respective boss is defeated.

Defeating any single mechanical boss for the first time causes 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 start to occur.
 * 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.
 * 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.
 * Reapers will spawn after just one mechanical boss is defeated.

Defeating all three mechanical bosses causes the following to happen:
 * The status message "The jungle grows restless..." appears.
 * Plantera's Bulbs begin spawning in the Underground Jungle, allowing the boss to be summoned.
 * Reapers and 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 1.4, crates acquired pre-Hardmode will no longer 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 two) 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 seem to shoot a red death laser when in at least one of their forms.
 * They are the only bosses whose Expert-exclusive items are crafting materials.
 * It seems that all mechanical bosses are made of hallowed bars since they drop them once destroyed.
 * Hallowed Bars, dropped by the mechanical bosses, are the one of the only bars that don't have a corresponding ore, the others being Spectre Bars and Shroomite Bars.
 * The Twins are the only mechanical bosses with no visible mechanical attachments until Retinazer's 2nd phase.
 * The Twins are the only mechanical bosses with no visible mechanical attachments until Retinazer's 2nd phase.

History
Boss mécaniques