The Fall was an short, though not instant, collapse of the old Kingdom of Tyranel, resulting in the destruction of many of its greatest cities and wonders, as well as total depopulation of its lands (including 60% dead, and the remaining 40% becoming refugees). The Orm began a campaign of burning cities to the ground, striking seemingly randomly across the entire territory. Eventually, he struck even at the capital city, though by that time is was empty, its population fleeing to found the New Kingdom in the south under the leadership of the last king.
During the ~1 year this campaign took, strange extraplanar creatures began to appear in large packs throughout the kingdom, accelerating the need to escape even as they impeded it, slaughtering many. These were seemingly ruled by the regent, a warlock who used the Orm's power to strike fear into the hearts of the people.
Despite the terrible violence of The Fall, neither the Orm, the regent, nor any of his minions ever left the lands of the Kingdom.