All about next year's book from the Black Library...

The seventy Loyalists led by Captain Garro commandeered the Imperial frigate Eisenstein and, evading the forces of Horus, were able to escape from the Istvaan system into the Immaterium. The Eisenstein was badly damaged, all its astropaths were dead, and its lone navigator mortally wounded. However, Garro managed to attract the attention of passing Loyalist ships by setting the Warp engines to self-destruct and ejecting them from the ship. Rogal Dorn had been becalmed in the Warp with his fleet for some time, and his navigators sensed the detonation of the Eisenstein's Warp drives. Making an immediate course for the location of the beacon, Dorn met with Garro, who explained to him all that had happened.
The Eisenstein was able to reach Terra, allowing the loyal Marines to report the extent of the atrocities that occurred in the Istvaan system. It was said that without this warning, the Imperium would have been in even worse shape to respond to Horus' next moves.