The Retribution is a vague, almighty force sealed away within the land of Riviera that had the power to completely annihilate it; it was presumably put in place by Asgard in the event of a future emergency. During the events of Riviera: The Promised Land, Ein and Ledah were sent on a mission to actuate the Retribution, as the Magi feared the resurgence of demons in Riviera as a catalyst for another Ragnarok. Ursula, Riviera's protector, took measures to prevent this at all costs, abducting Ein in the process and thus turning the tides in her favor.
Initiating the Retribution involved the unbinding of Seth, which required countless soul sacrifices.
It could be that Seth herself is the destructive power capable of destroying the land of Riviera, though it is never made 100% clear what it really is.