Mechanical bosses

Mechanical Boss is a term which refers to any of the three Hardmode bosses that can be summoned by "Mechanical" items, which are listed below. Once Hardmode begins, they will begin to spawn randomly until each individual one is defeated at least once. There is a 10% chance that one will be spawned each night, only condition being destruction of a single Demon/Crimson Altar.

Killing a Mechanical Boss will meet the requirements necessary for the Steampunker NPC to arrive and the Truffle NPC to sell the Mushroom Spear and Hammush. This will also cause Red Devils and Lava Bats to spawn in the Underworld.

After all three Mechanical bosses have been slain at least once each, the message "The jungle grows restless" will appear, and Plantera's Bulb will begin to grow in the Underground Jungle.