Josh Reynolds
Fury of Gork
Realmgate Wars (series) part 7

Still in search of allies for their war against Chaos, the Stormcasts brave the Ghurlands... but someone else is there, a champion of Tzeentch seeking a mighty weapon. Only a dangerous mission into the Howling Labyrinth can stop the sorcerer before it's too late.
Age of Sigmar
novel

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