 The embittered Iron Warriors have aligned with Horus out of frustration with the ignominious and unheralded role assigned them by the Imperium; before the story begins, they lash out in unforgivable genocide.In the story, they are invited to the freelance operation (which is unknown to the Warmaster) by Fulgrim and his Legion, who have their own agenda.The operation's true goal is kept secret from Perturabo and his Space Marines, who come to realise that not all fellow rebels can be trusted.