Matt Westbrook
Bladestorm
Realmgate Wars (series) part 8

Thostos Bladestorm and the Celestial Vindicators battle the forces of Chaos in the Realm of Beasts and grapple with the truth of the Stormcasts' immortality.
Age of Sigmar
novel

Also in:
The Realmgate Wars: the Novels
The Realmgate Wars: Volume 2