The Walker in Fire

The hive world of Sarastus harbours many secrets, and now many fugitives from the Imperium as well – a rogue Adeptus Mechanicus priest has absconded with several xenos biologis samples, prompting the Inquisition to order an intervention by the Deathwatch. The Salamanders Firedrake Garran Branatar forms part of a specialist Terminator kill-team sent to delve into the darkness beneath Sarastus, bringing the flame and fury of his Chapter in equal measure. But Branatar brings with him the shadowed legacy of his own past as well...

Read it because
It's Terminators in a hive world, hunting a tech-priest, and the main character has a secret that could affect the mission.