Dagon is a deity who presides over the Deep Ones, an amphibious humanoid race that currently resides in the Earth's oceans. He is first introduced in Lovecraft's short story "Dagon", and is mentioned extensively throughout the mythos.
Dagon is an enormous specimen of a Deep One that has been mentioned in texts since at least the time of Mesopotamia. He is worshipped by a devout cult of both humans and Deep Ones. People of Innsmouther always seek a sacrifice to appease Dagon.