Merrow on the Material Plane lived in undersea caves. They filled their abodes with treasures and used the rotted corpses of dead creatures tied to kelp to mark their territory.
A merrow was often armed with a harpoon. It also fought 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.