The Elder Scrolls: The Lord of Souls

The Elder Scrolls book The Lord of Souls By Greg Keyes

Description:
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?

Based on the award-winning Elder Scrolls® series, Lord of Souls is the second of two exhilarating novels that continue the story from The Elder Scrolls IV: Oblivion, named 2006 Game of the Year by numerous outlets, including Spike TV, the Golden Joystick Awards, and the Associated Press.

By the way, I personally love this book. Read the Infernal City too, it's the first book.