The Order of Dagon, also known as the Esoteric Order of Dagon, is an evil cult devoted to the Deep Ones, specifically the one known as Dagon - it is especially prominent in the port of the dreaded ghost town of Innsmouth and many of its members are corrupted by the Deep Ones and are willing to murder in order to keep power within the community.
The Esoteric Order of Dagon was the primary religion in Innsmouth after Captain Obed Marsh returned from the South Seas with the dark religion circa 1838. It quickly took root due to its promises of expensive gold artifacts and fish, which were desired by the primarily-fishing town.
The central beings worshipped by the Order were the Deep Ones, Father Dagon, Mother Hydra, and, to a lesser extent, Cthulhu. The Deep Ones were seen largely as intermediaries between the various gods, rather than as gods themselves. Even so, the cultists sacrificed various locals to the Deep Ones at specific times in exchange for a limitless supply of gold and fish.
The Esoteric Order of Dagon had three oaths which members had to take. The first was an oath of secrecy, the second, an oath of loyalty, and the third, an oath to marry a Deep One and bear or sire its child.
The Order was seemingly destroyed when one of Obed Marsh's lost descendants sent the U.S. Treasury Department to seize the town. As a result, the town was more or less destroyed, and the Order was thought disbanded.