Dragon Heist / Dungeon of the Mad Mage

Merrow are undersea merfolk-like monsters from the Abyss who terrorize the waters.

A merrow is often armed with a harpoon. It also fights with its sharp claws and its fanged bite.

Merrow were created when a tribe of merfolk found an idol to Demogorgon that drove them all insane. The king of this tribe demanded a blood sacrifice to the idol that opened a rift to the Abyss. The corrupted merfolk slowly transformed into the monsters now known as merrow over multiple generations there.