Lord of Souls — Greg Keyes

Forty years after the Oblivion crisis, the empire of Tamriel is threatened by a mysterious floating city, Umbriel, whose shadow spawns a terrifying undead army.

Reeling from a devastating discovery, Prince Attrebus continues on his seemingly doomed quest to obtain a magic sword that holds the key to destroying the deadly invaders. Meanwhile, in the Imperial City, the spy Colin finds evidence of betrayal at the heart of the empire -- if his own heart doesn't betray him first. And Annaïg, trapped in Umbriel itself, has become a slave to its dark lord and his insatiable hunger for souls.

How can these three unlikely heroes save Tamriel when they cannot even save themselves?