Mechanical bosses

Main article: 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, Eye of Cthulhu, and Skeletron, respectively. In addition, there is a 10*1/10 (10%) chance that one will randomly spawn each dusk 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. (i.e. 3.33*1/30 (3.33%) chance for one specific boss to spawn when none is defeated, then 5*1/20 (5%) chance after defeating one of them, finally 10*1/10 (10%) chance after defeating two).

The mechanical bosses' summoning items can be crafted at a Mythril or Orichalcum Anvil. They also drop from all Hardmode enemies (except statue-spawned enemies, Meteor Heads, and Old One's Army enemies) with a 0.04*1/2500 (0.04%) chance until their respective boss is defeated.

Defeating any single mechanical boss for the first time will cause 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 will now occur naturally.
• 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  Old-gen console,  Windows Phone, and 3DS 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 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, allowing Plantera to be summoned.

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 Mothrons now spawn during Solar Eclipses.

Boss		Spawn message	Summoning item		Dropped soul type
	The Destroyer	"You feel vibrations from deep below..."		Mechanical Worm	Soul of Might
	The Twins	"This is going to be a terrible night..."		Mechanical Eye	Soul of Sight
	Skeletron Prime	"The air is getting colder around you..."		Mechanical Skull	Soul of Fright

Crafting

Summoning items recipes

Result	Ingredients	Crafting station
Mechanical Eye	Lens3 Any Iron Bar5 Soul of Light6	Mythril Anvil or Orichalcum Anvil
Mechanical Skull	Bone30 Any Iron Bar5 Soul of Light3 Soul of Night3
Mechanical Worm	Rotten Chunk6 Any Iron Bar5 Soul of Night6
	Vertebra6 Any Iron Bar5 Soul of Night6

Notes

• There is a 10*1/10 (10%) chance for a random mechanical boss the player has not defeated yet to spawn naturally at dusk.
• When spawning naturally, a mechanical boss spawns exactly 81 seconds after the status message has appeared. Being in a layer lower than the Surface at that moment, or, in singleplayer, exiting the world during that timespan, will prevent the boss from spawning naturally.

• It is possible to summon a mechanical boss pre-Hardmode with a summoning item brought in from another world. However, if the Wall of Flesh has not been defeated, Plantera's Bulbs will not start spawning.

Achievements

Buckets of Bolts • “Defeat the three nocturnal mechanical menaces: the Twins, the Destroyer, and Skeletron Prime.”

Defeat all three Mechanical Bosses.

Category: Slayer

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.

Category: Challenger

Back for Second • “You have defeated all the prime bosses twice!”

Defeat all of the Mechanical bosses twice.

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 not give Hardmode rewards.
• On the  Windows Phone version and Nintendo 3DS version, 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  Old-gen console version,  Windows Phone version, and Nintendo 3DS version, it is possible to obtain a Terra Blade after defeating a single mechanical boss. Note that three Broken Hero Swords (As opposed to one) 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 shoot a red Death Laser: the Destroyer's segments, Retinazer in its second phase, and the Prime Laser.
• They are the only bosses whose Expert-exclusive items are crafting materials.
•  – According to the official game lore, the mechanical bosses are likely the parts of Cthulhu which the Mechanic was forced to build. This explains the lack of a mechanical counterpart of the Brain of Cthulhu, as the mechanical Cthulhu lacks a brain and the Mechanic was never able to finish it.

History

See the individual mechanical bosses' articles for more specific changes.

 PC version

• Desktop 1.3.0.1:
  • Life Fruit now only spawns after defeating a mechanical boss.
  • Now drop Mechanical Cart pieces in Expert Mode.

• Desktop 1.2.4: Fixed bug where "The jungle grows restless" message would appear before all three mechanical bosses have been beaten.

• Desktop 1.2.3.1: No longer spawn naturally if another boss is alive. Multiple bosses can still be summoned manually.

• Desktop 1.2.3:
  • Now slightly more difficult.
  • Plantera's Bulbs now only spawn after defeating all mechanical bosses.
  • The Mushroom Spear and Hammush are now only sold after defeating any mechanical boss.

• Desktop 1.2.1.1: Will no longer spawn during Pumpkin Moons.

• Desktop 1.2.1: Reduced the chance for a mechanical boss summoning item to drop during Hardmode.

• Desktop 1.2.0.3:
  • 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.

• Desktop 1.2: Now have a 10*1/10 (10%) chance to spawn at dusk until defeated for the first time.

• Desktop 1.1: Introduced.

 Console version

• Console-Release: Introduced.

 Mobile version

• Mobile 1.1.5822: Enabled & unlocked all Hardmode content.

• Mobile 1.1.8: Added all Hardmode content^{(Unobtainable)}

Nintendo 3DS version

• 3DS-Release: Introduced.