Chris Wraight
The Gates of Azyr

Korghos Khul rampages across the fiery Realm of Aqshy, hunting down mortal kind to slaughter or subjugate to Khorne. His Goretide have crushed all resistance... until the storm. Sent by Sigmar, the Stormcast Eternals have come to liberate all the realms from the yoke of Chaos.
Age of Sigmar
novel

Also in:
The Realmgate Wars: Volume 1
The Realmgate Wars: the Novels
Mighty Battles