The Word of the Silent King (Black Library Advent Calendar 2014 #13)

The Blood Angels and necrons, locked in a long and bitter war for the fate of the Gehenna system, are both endangered by the arrival of the monstrous tyranids. When the Silent King himself makes overtures of alliance, Commander Dante of the Blood Angels enacts a dangerous plan that could rid the Imperium of the Silent King forever - but when the two leaders meet at Devil's Crag, everything changes. Read it because
One of Warhammer 40,000's great mysteries is explored. Why did The Blood Angels ally with the ancient evil of the necrons at Devil's Crag? And maybe more to the point, why didn't the tireless aliens turn on the Space Marines when the battle was won? This story may have the answers.