Dark Adeptus

Dark Adeptus - Ben Counter A long vanished planet materialises out of the warp as over at the Eye of Terror, the 13th Crusade is well under way.
The Inquisition and a small team of Grey Knights go to investigate, but the Adeptus Mechanicus also want to get involved.

The small group manages to get even smaller as various members are killed off quite brutally.
The author does a good job of playing up the teams different reasons for being there, making it hard to know who can be trusted, or who is working towards an ulterior goal.