Fight against Duke Fishron
Note: This requires v1.2.4. Get a Golden Fishing Rod from Pirates in the Hardmode Ocean Biome. Either use a Treasure Map or wait for a Pirate Invasion (happens only after the Wall of Flesh is defeated). Get a Truffle Worm in a Mushroom Biome. They appear as small glowing worms and can be captured by quickly dropping down on it while swinging the bug net. Travel to the far east or west oceans and use the Golden Fishing Rod with the Truffle Worm as bait. Duke Fishron should appear and attack.