Following his sons' designs on the title of Warmaster, the Emperor declared that there shall be no Warmaster to replace Horus. Both Lion El'Jonson and Roboute Guilliman reacted badly to this, temporarily ceasing joined military operations and isolating themselves. After secluding himself and his Legion following the Emperor's decree, Lion El'Jonson and Kor Phaeron decided that the time had come to act upon their greatest potential ally, Roboute Guilliman. With the help of him and his Ultramarines, they could finally declare open war on the Imperium.
Together, Lion El'Jonson and Kor Phaeron devised a secret plan to turn Guilliman. They turned to the eminent Sorceror Zahariel, who had previously thwarted an assassination attempt on the Emperor himself, and had his memory of the incident suppressed by the Emperor. Since the Lion's fall to Chaos, Kor Phaeron had been instructed by the Gods to examine Zahariel for "forgotten secrets." Zahariel's memory was restored through sorce