During the second day of the attack on Tavos IV, the defending forces of the imperium were struggling to repel the attack from the vile forces of chaos, they had successfully taken down the chaos lord (not before he had managed to miss with his axe and whack himself in the face) but still the chaos horde rolled on, with orks and eldar in the rear.
The attackers had made short work of the defenders front line (even though they had to constantly stop and check the toughness of eldar bikes and the front armour of a fire prism). With defeat looking certain our Librarian gets the bright idea to use some of his sanctioned powers in a last ditch attempt of killing a few more of the attackers.
It is in this moment that the emperor decided he had had enough of this particular individual and so when he rolled his leadership, the Emperor sent an almighty psychic slap through the warp, causing the Librarian to fail his roll. The slap was so hard it took out the Librarian and 4 wimps of Guilliman who were cowering in close proximity.