The Face of Treachery (The Horus Heresy #Short Story)

The Raven Guard have arrived at Isstvan V to rescue Corax and other survivors of the treacherous Dropsite Massacre, but the enemy still encircle the world. As starships clash in fierce void war and the life of a primarch hangs in the balance, the key to victory lies in the hands of one Space Marine in the rebel fleet, a warrior who is not what he appears.

Read it because
It bridges 'Raven's Flight' and Deliverance Lost, showing the trials of Commander Branne's fleet as it tries to rescue the primarch Corax from the Isstvan system. And it shows once again that the Alpha Legion are simply everywhere...