Mechanical bosses

The Mechanical Bosses are three Hardmode Bosses that 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. Each also drop from all Hardmode enemies (except statue-spawned enemies and Meteor Heads) with a 1/2500 chance until their respective boss is defeated.

Once any single Mechanical Boss has been defeated for the first time:
 * 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.
 * Yelets 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 Code 2
 * Old One's Army 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.

After all three Mechanical Bosses are defeated:
 * The status message "The jungle grows restless..."
 * Plantera's Bulbs begin spawning in the Underground Jungle. The breaking of these is required to summon Plantera, which is required to have the ability to enter the Jungle Temple.
 * Reapers and now spawn during Solar Eclipses (on, Reapers will spawn after just one Mechanical Boss is defeated).

Tips

 * Players who aren't confident in their ability to defeat the bosses, but who want access to the Hardmode Ores, can refrain from destroying any Altars until they are ready and instead fish for Crates, which offer those ores and their bars as random rewards during Hardmode. Crates acquired pre-Hardmode can also be saved until Hardmode, and will still offer Hardmode rewards.
 * Once one of the spawning messages appears at random, fleeing underground, or logging out and back in again, can prevent the Mechanical Bosses from spawning.
 * On the and, the player can find a Drax in an Underworld Shadow Chest, allowing the player to get Turtle armor before defeating any of the Mechanical Bosses, making it much easier to defeat them on the first try.
 * On the, and , if you’ve successfully defeated a mechanical boss you may want to make the Excalibur and then live through a Solar Eclipse. If you’ve obtained a Broken Hero Sword craft a True Excalibur at a Mythril/Orichalium anvil. If you have the Night's Edge and you have another Broken Hero Sword from the Solar Eclipse craft a True Night's Edge and use that instead. If you have another Broken Hero Sword use the True Night's Edge and the True Excalibur with another Broken Hero Sword you can craft the Terra Blade.

Trivia

 * Some teasers for the release included The Destroyer having blue-lit segments instead of red.

History
Boss mécaniques