Saul Tarvitz approaches you as you cleanse Chorral city on Istvaan III. he has a wild look about him as he runs towards you. When he gets within earshot, his words come out as a tumble

"Horus has turned traitor! Fulgrim, Angron, Mortarion, everyone currently in orbit, they're traitor. They're planning to betray everyone currently deployed on Istvaan III and virus bomb us to death. Get into a bunker and spread the word!"