Graham McNeill
Dead Sky, Black Sun
Chronicles of Uriel Ventris (series) part 3

Exiled Ultramarines Uriel Ventris and Pasanius venture into the heart of the Iron Warriors’ fortress-world in search of atonement – but will they find anything but horror and death?
Warhammer 40k
novel

Also in:
The Uriel Ventris Chronicles: Volume 1
Honour And Vengeance: Uriel Ventris Vs Honsou