The Time Before Time
In the distant past the world was ruled by two great human factions of great users of magical weaponry. These empires warred nearly constantly in a near stalemate, then one began to gradually push the other back, until it was only able to control a small portion of its previous empire.
This lead to the cataclysm. The nearly vanquished empire unleashed the fury of its most feared weaponry that literally scorched the planet. The other empire responded in kind and night fell over the whole world.
Millenia passed and life once again began to crawl out of its hiding places. Creatures began to hunt and move across the scarred landscapes until small groups began to form and coalesce into tribes of like minded beings. These tribes eventually began to settle down into defensible areas and develop culture. Then as histories began to be written the previous era becam known as the time before time.