CL Werner
Lord of Undeath
Realmgate Wars (series) part 10

Many attempts have been made by Sigmar's Stormcasts to forge an alliance with the God of Death, mighty Nagash. The latest attempt sees the Anvils of the Heldenhammer enter the Undying King's domain, with a desperate plea for aid…
Age of Sigmar
novel

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