Damnos

When Damnos is hit by cataclysmic earthquakes, an ancient force is awakened. Deep beneath the earth, the necrons rise from their slumber to decimate the human populace. All appears lost until salvation comes from the heavens - the Ultramarines brave an orbital bombardment to deploy their forces on Damnos, led by two legendary warriors - Captain Cato Sicarius and Chief Librarian Tigurius. They are the planet's last, great hope against the remorseless alien foes, but tensions within their ranks threaten to derail victory. As battle rages on Damnos, and the Ultramarines seek to defeat their soulless enemies, Tigurius receives a terrible vision a vision telling of the death of a hero...

Read it because
It's a truly epic tale of the Ultramarines finding themselves outclassed by a mysterious enemy and struggling to survive in the face of ancient horror and hate...