Truffle Worms can be used as bait for Fishing, and are listed at 666% bait power. When a player fishes with a Truffle Worm in the ocean, Duke Fishron will be summoned (100% chance). Fishing anywhere else using this bait will not catch anything.
666 is commonly known as the Number of the Beast (it should be 616) , probably an allusion to that fishing with a Truffle Worm will summon Duke Fishron.
When the Truffle Worm is in the player's inventory, the Fisherman's Pocket Guide (and its combined accessories) will display your fishing power simply as "Warning!"