Father Dagon

"'Then suddenly I saw it. With only a slight churning to mark its rise to the surface, the thing slid into view above the dark waters. Vast, Polyphemus-like, and loathsome, it darted like a stupendous monster of nightmares to the monolith, about which it flung its gigantic scaly arms, the while bowed its hideous head and gave vent to certain measured sounds. I think I went mad then.'"- H. P. Lovecraft, "Dagon"

Father Dagon is a central deity of the Esoteric Order of Dagon based in the town of Innsmouth. He is one of the two reigning monarch-gods of the Deep Ones, along his consort Mother Hydra.

Origins
The origin of Dagon is rather ambiguous and it is never clearly stated if he is  a Great Old One (like Cthulhu) or simply a large and very powerful Deep One. It is also unclear whether he is native to Earth or was if he was (along with other Deep Ones) part of the invasion force that came to Earth alongside Cthulhu and his Star Spawn.

Throughout the ages though, Dagon gathered a large following, ultimately being worshipped by many Oceanic tribes, such as the Kanakies and even ancient Mesopotamian cultures. He is mentioned in a book by Captain Abner Ezekiel Hoag, written in 1734, called Ponape.

Inspiration:
The etymology of the name is most likely derived from the Ugaritic word "Dgn" or Akkadian "Dagana". The Ugaritic meaning of the word is "grain". It is believed that he was worshipped among the people of: Ebla, Assyria, Ugarit and Amorites.

He is also mentioned in the Bible, where he is associated as the Philistines god of fish. This led to many associating him as a "fish-god" and "merman-god". Many scholars agree however that he was worshipped as the god of fertility and agriculture.

Lovecraft's interest in Arabic and Oriental culture most likely led to him incorporating the ancient god into the Cthulhu Mythos.

Role in the game
Dagon is referenced numerous times throughout the game, especially in the chants by the followers of the Order who say "I'a Dagon". He makes an appearance in the later part of the game during the sequence "The Dangerous Voyage" in which, at the very end, Dagon attacks the Cutter Urania with Jack still on board.

Using the ship's main canon, Jack eventually defeats Father Dagon, but whether he was killed by the encounter or just wounded remains uncertain. It is later revealed he was summoned by Robert Marsh on Devil's Reef prior to his attack on the ship.

Trivia:

 * One of Lovecraft's first better-known stories was "Dagon" (1917)
 * There is a horror film inspired by "The Shadow over Innsmouth" called "Dagon".