Seth is a minor antagonist in Riviera: The Promised Land and is the counterpart of Ursula; she has been branded "The Sealed One", or "The Sprite of Death". Seth is actually a child who innocently craves power, and had been sealed away within the Maze of Shadows (presumably by the Gods) because of her violent, destructive nature. She was summoned by Hector at the cost of many souls, the final ones necessary being that of Malice and the girl whom Ein cares for the most. Hector's ambition was to merge with her and become an almighty entity known as Seth-Rah.
Seth herself does not actually do anything, and she appears very briefly before Hector forcibly fuses with her. As a result, Seth has not been fleshed out at all and very little is known about her origin, history, etc. Ursula's apparent link to Seth is also not explained. It is suggested in game that actuating the Retribution involves Seth's unbinding, which could mean that Seth herself is the destructive force that would destroy all of Riviera.