Merrow

(Long ago, a tribe of merfolk found an idol of Demogorgon at the bottom of the sea. Not knowing what it was, they brought the artifact to their king. Everyone who touched the idol became afflicted with madness, including the king, who decreed that a sacrificial ritual be performed to open a gateway to the Abyss. The ocean turned red with the blood of slaughtered merfolk, but the ritual worked, and the king led the survivors through the underwater gate to Demogorgon's layer of the Abyss. The merfolk remained there for generations, fighting for their lives as the Abyss wraped them completely, transforming them into hulking, evil monstrosities. Upon returning from the Abyss to our realm from the Underdark into the worlds oceans, they spread through conquest, raiding merfolk and sacrificing them to Demogorgon in return for greater power.)