Nixies

These aquatic first cousins of the ancient race of Elves rise from the depths to strike terror into the hearts of seafarers. The evil Nixies wield sharp tridents which can be hurled against a ship's crew from afar, causing great damage. They also can attack people at land, thus making them very dangerous.

They can be found, as said, at sea.

They appeared only in Ultima IV. After that, they seemed to either have been destroyed, or retreated deep under the sea.