Buckets of Bolts

All three mechanical bosses. Summons craft at a mythril/orichalcum anvil from souls and parts (they also spawn naturally at night until beaten): The Destroyer (Mechanical Worm) - a long sky bridge and piercing/AoE weapons shred its segments; The Twins (Mechanical Eye) - an asphalt or platform runway and sustained single-target fire, kill Retinazer or Spazmatism one at a time; Skeletron Prime (Mechanical Skull) - hands first or straight to the head, your call. Hallowed bars and souls from each unlock the next tier - and Drax Attax, Mecha Mayhem and Plantera beyond.