Iron Soul

Tracking a monstrous tyranid creature, Wolf Priest Jathrac Leatherhand finds the remains of several battle-brothers of the Iron Hands. As he gathers their gene-seed to return to the Chapter, he realises that one of them – encased in a mighty Dreadnought sarcophagus – yet lives. But Brother Radamarr is not all that he seems…

Read it because
It's very simply the creepiest and darkest look at the ultimate end of the Iron Hands' path that we can imagine.