The Chains of Heaven

From the introduction: At the roof of the world, the Seree constructed a fortress, an enchanted prison meant to wring magic from the gods themselves. The Seree fled centuries ago when a trio of demigods, led by Deel, razed it and freed their trapped kin. Now, the prison is controlled by the sorceress Nacharta, who claims Seree ancestry and seeks to reclaim the powers of her forebears.