Mechanical Boss is a term which refers to any of the three Hardmode bosses that can be summoned by "Mechanical" items which, besides being craftable, drop in hardmode until the respective boss is defeated. These items are listed below. Besides being summoned, there is a 10% chance that one will be spawned each night after the first Demon/Crimson Altar is destroyed. They will continue to spawn randomly until each individual one is defeated at least once, the only condition being destruction of a single Altar.