- Main article: Boss
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:
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.
- Bringing the Mechanical Bosses’ summoning items to a pre-Hardmode world makes it possible to defeat them prior to defeating the Wall of Flesh. However, the Wall must still be defeated in order to unlock Plantera.
- There is a tooltip mistake for the Mechanical Worm. The tooltip says "Summons Destroyer", but should actually say "Summons The Destroyer"
- 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 Mobile version and version, 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.
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- Some spoilers for the Mobile version release included The Destroyer having blue-lit segments instead of red segments.
|| Buckets of Bolts • "Defeat the three nocturnal mechanical menaces: the Twins, the Destroyer, and Skeletron Prime."
| Defeat all three Mechanical Bosses.
|| Mecha Mayhem • "Do battle against the Twins, the Destroyer, and Skeletron Prime simultaneously and emerge victorious."
| Defeat all three Mechanical Bosses simultaneously in a single battle.
|| Back for Second • "You have defeated all the prime bosses twice!"
| Defeat all of the Mechanical Bosses twice.
- See the individual mechanical boss articles for more specific changes.
- Desktop 1.2.4: Fixed bug where "The jungle grows restless" message would appear before you have beaten all 3 Mech Bosses.
- Desktop 18.104.22.168: Mech bosses no longer spawn randomly if another boss is alive. You can still summon multiple bosses manually.
- Desktop 1.2.1: Reduced the chance for a mechanical boss summon to drop during hardmode.
- Desktop 22.214.171.124:
- Hardmode Crimson, Corruption, and Hallow spread is now slowed after killing Plantera instead of any Mechanical Boss.
- Slightly increased the chance for a Solar Eclipse to happen, but they will now only happen after killing a Mechanical Boss.