Merfolk finding the idol of Demogorgon (D&D)

A long time ago, a group of rangers from a merfolk tribe found an idol of Demogorgon at the bottom of the sea. Anyone who touched it would go mad, but not knowing of its power they brought the artifact to their king. The monarch, deranged, ordered the ritual sacrifice of many of his subjects to open a gateway to the Abyss, where this demon dwelled, and led the survivors there.