Duke Fishron

Duke Fishron is a Hardmode boss that is summoned at the Ocean Biome. It is immune to the On Fire!, Poisoned, Cursed Inferno, and Frostburn debuffs. To summon this boss you first need to obtain the Truffle Worm. After obtaining it, go to the west or east ocean (it doesn't matter which one), and then cast any Fishing Rod into the ocean, with the Truffle Worm as the bait. When you get a bite, reel it in, and Duke Fishron will spawn and attack you.