Mechanical bosses

The term "mechanical bosses" refers to the 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. 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 will not spawn until Golem is defeated).
 * Plantera's Bulbs begin spawning in the Underground Jungle.

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.
 * Reapers and now spawn during Solar Eclipses (on, Reapers will spawn after just one mechanical boss is defeated).

Skeletron Prime
(The recipes are different on old gen console version)

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. Crates acquired pre-Hardmode can also be saved until Hardmode, and will still offer 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 several Broken Hero Swords 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.

History
Boss mécaniques