Sigmar had travelled the realms for many years, seeking allies of power to protect the mortal realms from the forces of Chaos and help build a new utopia. He first gathered them in his own Realm of Azyr, leveliing the top of Mount Celestian to provide a setting for the first Grand Council.
At this council, eight protectors of the Realms were appointed and for a time all was well. Nagash imposed order on the dead and his deadwalkers were prolific builders, Grungni taught mankind the metalcraft and under their benevolent rule, trade and civilisation flourished. Gorkamorka was given the task of clearing the wild countries as a monster hunter and set to the task with gusto and joy. The few incursions by Chaos in this period are repulsed by the united forces of the Pantheon. 
Yet it would not last, for each of the members had their own priorities or ambitions, Alarielle grew tired of the bickering and her visits grew evermore infrequent whilst Gorkamorka threw off the tedious restrictions of the other members of the pantheon and began a great destructive Waaagh! across the entire mortal realms. The alliance was treated as peripheral by Tyrion and actively subverted by Malerion. Slowly the alliance fell apart and by the time Chaos erupted into reality to attack the Allpoints, it was little more than a name. It truly ended when Sigmar hunted down Nagash after what seems to be a betrayal, during a war that would be known as the War of Heaven and the Underworlds.