The Age of Chaos is the long era of catastrophe that preceded the current epoch of the Mortal Realms. It began when the Dark Gods, having long coveted the realms' bounty, poured their daemonic legions and mortal worshippers through corrupted realmgates and shattered the golden civilisations of the earlier Age of Myth.
The pantheon of gods that had once ruled together fractured under the onslaught. Alliances collapsed amid betrayal and blame, deities were slain, driven mad, or scattered, and one by one the realms fell to slaughter, enslavement, and forced worship of the invaders. Sigmar fought longer than most, but after crushing defeats he made his most controversial choice: he withdrew every force he could gather to Azyr and sealed its gates, leaving the remaining realms to endure generations of darkness alone.
For those outside, the era meant ruined cities, hunted peoples, and survival in hiding, and its scars define nearly every culture in the setting. Yet the isolation also allowed Sigmar to prepare his answer. When the Gates of Azyr finally reopened and the Stormcast Eternals descended on bolts of lightning, the Age of Chaos ended and the Age of Sigmar, the era of reconquest, began.