161.000 years ago Tzeentch created a new Magic spell which amused him. If used, it would allow a sorcerer masterfully enough to use it to undo one previously released magic power. It was written down in the Book of Changes and hid in a vault on a deathworld covered by eternally erupting volcanoes named Nebustion. Zit was buried deep down under the ash covered surface. Its existence was forgotten except by a few in service of Tzeentch. Eventually humans spread out between the stars, discovered Nebustion and placed an imperial force on it to guard till it could be properly investigated.

Now a mighty Herald of Tzeentch was captured by the Nurgle Daemon Prince Yyzz, Master of the Flies, and among the information gained from herold, the existence and location of the Book of Change was revealed. Yyzz sensed a golden opportunity to trap more servants of Tzeentch with this knowledge, and maybe even one of the greater ones. Many years were spent creating a devious and cunning plan. When all was ready, a messenger force was sent to find Ahriman and to ask his interest in the book. Yyzz offered to accompany him to the hiding place, of course with a minor force for his personal protection only.

It would be a long a dangerous journey, so Yyzz suggested that Ahriman would bring small retinue on his own. If the artefact could be recovered, Ahriman be required to join Yyzz in an quest of his own. Of course, Yyzz did not except the sorcerer and his servants to leave Nebustion alive, but if they did there would still be numerous enemies to combat before the quest would be over.

Ahriman, upon learning of the book and seeing a chance to undo his greatest error and curse, releasing the Rubicon. Knowing it to be trap, Ahriman accepted the invitation while bringing a larger retinue than he had for as long as he cared to remember. There was little chance of success, but he could not ignore even the smallest chance of undoing what he had done.

Yyzz would certainly abandon the strange alliance when convenient, but not as long as his security was in danger from the human soldiers or what else. Battle could not be avoided, and if not the preparations were enough, death or banishment waited for all.

Thus the awkward alliance left for Nebustion.

Exactly as planned.