Dagon, originally known as Dagan, was a major northwest Semitic god, chiefly of grain and agriculture, though he possibly had other functions as well, for example as a deity involved in funerary rites. He was worshiped by the early Amorites and by the inhabitants of cities such as Ebla, Ugarit and especially Tuttul, where he was the tutelary god. He was known as the "Father of Gods," with his more prominent children being the storm god Adad (the Ugaritic Baal, often addressed as "Son of Dagan" in the Baal cycle) and the goddess Hebat. His wife was usually Shalash, who was also an agricultural goddess. In Mesopotamia, he was sometimes viewed as a member of Enlil's entourage. He was seemingly equated with the Hurrian and Hittite god Kumarbi, also associated with grain, and also considered to be Shalash's husband sometimes (however, Kumarbi had a hostile relationship with the corresponding storm god). Some scholars also suggest he was conflated with Ugaritic El, though this contradicts the description of Dagan as the god of Tuttul, common in Ugaritic texts.
In later times, Dagon was also a major member, or perhaps head, of the pantheon of the Biblical Philistines, and in this role is also known under the name Marnas. The Bible, in an attempt to vilify the opposing religion, narrates that his temple was destroyed by a human sacrifice for him, Samson. Eventually, Dagon became a part of Abrahamic demonology, being sometimes portrayed as a high ranking demon, for example, in Paradise Lost.
Dagan was not a fish god. The XIXth century association between the name Dagon and the so-called Assyrian mermen is the result of an incorrect etymology proposed by Bible commentators in the Middle Ages.
Dagon's appearance as a sea monster was likely inspired by H.P. Lovecraft's depiction of the figure, which was influenced by the outdated view of this deity as a fish creature. In Lovecraft's books, he is a deity who presides over the Deep Ones, an amphibious humanoid race indigenous to Earth's oceans. He's also known as Father Dagon and the consort of Mother Hydra. Although they are deities, they are generally not considered Great Old Ones. He is worshipped by the Esoteric Order of Dagon, a secret cult based in Innsmouth.
"Origin: Babylonia. An ocean god worshipped by the Philistines. It's said his name comes from the Hebrew sounds for fish (dag) and idon (aon). There used to be important temples for Dagon in Gaza and Ashdod. Dagon is identified as the same as the Babylonian god Oannes. He is depicted with a male appearance, with a human's upper body and a fish's lower body."
—Shin Megami Tensei A-Mode DDS dictionary
Persona 2: Eternal Punishment
Dagon resides in the Tower of Perception that exists in the Kadath Mandala. Zula warns the group not to listen to him chant or they would be converted to Cthulhu's cult. Both Ulala and Katsuya are temporarily converted and the party fights against Dagon. Defeating him and watching him disappear, leaving the Trapezohedron of Perception behind.
DemiKids Light/Dark Version
"A resurrected mermaid-demon with unknown powers that have yet to be awakened."