Duke Fishron, the fish boss of Terraria, can be summoned by fishing in the Ocean using a Truffle Worm as bait. The Truffle Worm is a rare critter that you need to catch with the Bug Net, Golden Bug Net, or Lavaproof Bug Net in an underground Glowing Mushroom biome. Once the bobber bobs, reeling in the line spawns Duke Fishron. It's important to note that summoning Duke Fishron does not require that any other bosses have been defeated beforehand. He can also be summoned by fishing in lava or honey if within the Ocean biome. The player's Fishing Power is irrelevant to the summoning.