Leviathans are underwater Rahi. It is unknown if they exist in the main storyline universe; they have only been seen in an alternate one.
They possess venomous fins and teeth, but they are incapable of controlling the dosage administered to a victim. Their eyes are said to be hypnotic, though this is not a true power.
They are most often seen in the army of an alternate Leviathos. He uses them mostly for close-range combat.