Magic Eater can be fished out of the wishing well and sewers in Dalaran. Upon eating the fish, you will either receive a random high level Well Fed buff or a 45 second polymorph Wild Magic.
Polymorph Wild Magic
Documented forms include a basilisk, cockroach, dragon whelp, and wisp. You can continue to fish in this form and use items in your inventory, but you cannot cast spells or engage in combat.
- It may be possible that the Magic Eater along with the Sewer Carp are a reference to the Pokémon; Magikarp. This may be why they're the only fish caught in the sewers.