Sigmar unifies the Eight Realms and finds the deities who survived the destruction of the World Before Time. Cities were built and cultures grew under Sigmar's wise rule and prosperity and utopia was achieved briefly.
Chaos legions erupt from the void and engulf the Eight Realms in conflict. Sigmar forges alliances and fights Chaos but ultimately is forced to retreat to the celestial realm and the gates between realms are closed.
Chaos dominates 7 of the Eight Realms and turns them into places of living hell.
Sigmar divorced the Celestial Realm from the rest of the Eight Realms and brought one of his allies, Grungni the creator. Together they forged an army made up of warriors armed with Sigmarite weapons and armour from the World-That-Was. He formed the Stormcast Eternals. Only once his army was completed did Sigmar reopen the Gates of Azyr and unleash his army.