Mechanical bosses

Main article: Bosses

The term "mechanical bosses" refers to three early-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 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.

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 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(Desktop, Console and Mobile versions) can be dropped by any enemy in the Jungle biome.
• (Desktop, Console and Mobile versions) Life Fruit will begin to grow in the Underground Jungle (on the  Old-gen console 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(Desktop, Console and Mobile versions).
• (Desktop, Console and Mobile versions) The Dye Trader begins rewarding the player with Chlorophyte Dye, Living Ocean Dye, Living Flame Dye, and Living Rainbow Dye in exchange for Strange Plants.
• Old One's Army(Desktop, Console and Mobile versions) upgrades to Tier 2 and will progress to wave 7 (however, Betsy or Etherian Lightning Bugs will not spawn until Golem is defeated).
• (Desktop, Console and Mobile versions) Moss Hornets and Moths gain a chance to drop the Tattered Bee Wing and Butterfly Dust respectively.
• (3DS version) 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, indicating that (Desktop, Console, Old-gen console and Mobile versions) Plantera's Bulbs have begun growing in the Underground Jungle. Breaking one of these bulbs summons Plantera.
  • On the  Desktop version,  Console version, and  Mobile version, one Plantera's Bulb immediately spawns once all three mechanical bosses are defeated for the first time.
• (Desktop, Console and Mobile versions) Reapers now spawn during the Solar Eclipse.
• (Desktop, Console and Mobile versions) The Traveling Merchant will begin to sell the Pulse Bow.

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

• When spawning naturally, a mechanical boss spawns exactly 81 in-game minutes (at 8:51 PM) 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.
• With the 1/10 (10%) chance of spawning naturally at dusk, the resulting chance for each mechanical boss is initially 1/30 (3.33%). Once one of them has been defeated, the remaining two have a resulting 1/20 (5%) chance of spawning naturally, and after defeating the second one, the last one has the full chance of 1/10 (10%).
• An Eye of Cthulhu naturally spawning will take precedence over a mechanical boss naturally spawning.
• A Lantern Night(Desktop, Console and Mobile versions) occurring will prevent a mechanical boss from naturally spawning. However, summoning a mechanical boss using one of the summon items will cancel out the Lantern Night, just like any other boss summon.
• In a Get fixed boi(Desktop, Console and Mobile versions) world, the player cannot summon the mechanical bosses separately, they have to summon the Mechdusa instead.
• It is possible to summon a mechanical boss in pre-Hardmode if the summoning item is 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. (Desktop, Console and Mobile versions)

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. (Desktop, Console and Mobile versions)

Category: Challenger

Slayer of Worlds • “Defeat every boss in Terraria.”

Defeat every boss in the game. (Desktop, Console and Mobile versions)

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 1.4.0.1(Desktop, Console and Mobile versions), crates acquired in pre-Hardmode will not give Hardmode rewards.
• On the 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 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(Desktop, Console and Mobile versions)-exclusive items are crafting materials.
•  – According to the Terraria lore pages, 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.

 Desktop version

• Desktop 1.4.4: The Pulse Bow is now sold after defeating all mechanical bosses.

• 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 slightly less difficult.
  • Now have a 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, but they are unobtainable.

Nintendo 3DS version

• 3DS-Release: Introduced.