Dagon

Dagon was a demon lord, also called the Prince of the Depths. His realm was the 89 layer of the Abyss, and was known as the Shadowsea.

Description
Dagon's form was a mixture of both octopus and fish. Dagon sprouted countless tentacles and was covered in black, rubbery skin that was punctuated with many red, unblinking eyes. Fish scales shielded his flesh, and row upon row of long fangs jutte from his maw. He stood more than tall and was considered one of the most fearsome of the demon lords.

History
Dagon's history on Toril lay below the surface of the Sea of Fallen Stars and the waters in the west of Maztica. He and his minions lurked for a long time in the Trench of Lopok and were only known as "Those who Sleep Below". They infested the dreams of those who swam in the Sea of Fallen Stars with nightmares. The most ambitious of the demons was Prince Dagon, who planned to overcome a pantheon of sea creatures and achieve their divine powers. To reach this goal he brought the depths of the Sea of Fallen Stars under the control of his servants and their forces. The next step was made with long-forgotten eldritch magic. He restricted the influence of the deities of the shalarin in the Sea of Corynactis, on the west of Maztica.

The next step was the creation of five "wild tides" to sweep large numbers of shalarin into the Sea of Fallen Stars, where his minions could slaughter them. The first wild tide began in with the opening of the Wildtide Portals connecting the two ocean realms. Many shalarin were swept into the Sea of Fallen Stars, where roughly 70% died under the attacks of Dagon's servants. The rest were only rescued by the intervention of enemies of Dagon, like merfolk.

Dagon repeated this tactic every 720 years and the last wild tide was in the. This time, the portal stayed and became permanent. The shalarin of Faerûn kept the portal a secret, fearing that others could use it for their own ends. But another fact was more disturbing for them. Between the fourth and fifth tide, the shalarin of their homelands near Maztica all but abandoned their gods because they had grown silent. Instead of the gods, they began to venerate the demon Dagon and his cult became the dominant religion in the Sea of Corynactis. The shalarin of the Sea of Fallen Stars cut off the travel between their communities. But it was too late and the cult of Dagon had already taken root among their ranks and grew in secret.

Cult of Dagon
Dagon was served by aquatic demons, especially wastrilith, and was worshiped by aquatic races such as kraken, chuuls, sea hags, water nagas, and kuo-toa.

Progeny
Dagon also seems to have had heroic, "good" progeny in Faerûn, one for certain in the form of Captain Aulruick Thoster, one of the heroes attempting to stop the eladrin Malyanna from opening the Far Manifold with the Key of Stars in the. Thoster's heritage manifested itself in increments periodically during his quest; originally, he believed it to be of kuo-toa origin, but eventually it was revealed to him (via an extremely painful transformation) that he was in actuality a 'demon scion' and a direct descendant of Dagon.

The Outcast Dagon
The demon lord Dagon should not be confused with the exiled devil of the same name, who dwelled on Avernus, first layer of the Nine Hells. Originally known as Jaqon, Asmodeus forcibly changed Jaqon's name to 'Dagon' to thwart attempts to summon the offender.