Overgods are the manifestations of the four prime Laws (Fate, Life, Destruction, and Death).
They never get involved in worldly matters and are the most powerful existences second only to an Universe Creator. They have set the rule that any Sovereign that destroys a Material Plane by entering it despite their incompatibility, will get destroyed.
Every so often they will issue an overgod missions and whoever completes the mission can make a request to the Overgod who issued it. (most ask for an Overgod artifact)