This story takes place during the Dark Imperium story arc. Mephiston, the bad-ass Chief Librarian of the Blood Angels, is searching for an artifact that is clouding his pskyer vision.
On the planet of Morsus, set adrift from Imperial control during the Great Rift expansion, a rag-tag band of Sabine Guard (Imperial Guard) are holding out a small Imperial bastion against hordes of malfunctioning Necrons. Mephiston goes on a soul-journey and finds the item amongst the Necrons and inspires the Sabine Guard into helping in the capture of the item. That's the gist of the excellent story and one you should read for yourself. There is a lot that happens. It seems rather the standard Astartes- see/ Astartes-kill story. Sort of. It's really about Mephiston and the changes he underwent.
Once Mephiston was simply Calistarius, but was gravely wounded in combat and left for dead. He managed to overcome his chapter's blood thirst and the blood rage, with the help of Sanguinius and the Emperor, to arise as Mephiston. Power like his is rare, even amongst Chapter Masters, and really reminds me of Ahriman of the Thousand Sons. His powers and visions are beyond the norm. It is an eye opening thing to read about.
The story of the Sabine Guard and it regaining some semblance of honor was well done, as was the surprisingly touching loyalty of the ogryn mutants. The story of Mephiston's quest, the Sabine Guard's redemption and the substory of (I won't spoil the name) a Blood Angel who falls to the subtle wiles of Chaos all had deep resonance and I really enjoyed it all.
Any 40K fan will really enjoy this great little addition. Consider me a fan of Mephiston. This guy is a serious bad-ass.