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 and Underground Corruption/Underground Hallow biomes.
 * The Demolitionist will begin selling Land Mine.
 * Life Fruit will begin to grow in the Underground Jungle (Life Fruit will already have been growing on the, but will now grow even more after a Mechanical Boss is defeated).
 * Old One's Army upgrades to Tier 2 and will progress to wave 7. Betsy will not spawn yet, however.

Defeating all three triggers the status message "The jungle grows restless...", and 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. This will also cause Mothrons and Reapers to spawn in the Solar Eclipses.

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.

Trivia

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

History
Boss Mécaniques