Many centuries ago, the powerful spherewalking mage Sharza-Zara found the realm of Underlight, and liked it enough that she spent a great deal of time there. Rather than simply enjoying the realm, she caused great improvement throughout, beginning with the education of the inhabitants and leading up to the shining achievement of the great City, named Stonerise.
But despite these great benefits, tension flared between the worshipers of the two gods, and war loomed on the horizon. Unable to bear her adopted children killing each other, Sharza-Zara cast the Spell of Peace and ended the conflict by force. As punishment and as a reminder, she also reversed the night and day of the realm, perhaps to teach each the understanding of the other.
Realizing that the inability to war could be a fatal weakness for Underlight in relations with other realms, Sharza-Zara cast the Spell of Isolation before her death (by natural causes; she may very well have possessed the power to remain immortal, but chosen not use it.) She now sleeps in a tomb inside The City.