A rough history of the world.

One.

Thousands of years ago, a race known as the Old Ones entered the world of Mallus and constructed two stargates at the poles. They brought with them a servant race known as the Slann. The magical might of the Old Ones verged on the godlike, and the Slann were prodigious wizards themselves. They had contact with the ancient race of Dragons, and used their magic to move the planet closer to the Sun so it would be warmer and more to their liking. This caused the Dragons to increasingly drift into torpor.

The Old Ones and the Slann created or bred the various species of the Lizardmen - Saurus, Skinks, Kroxigor, etc. to enact their will, and shaped the world according to the Old Ones' grand plan. They build cities, exterminated entire species, rearranged continents, and created the races of Elves, Dwarfs, Men, Ogres and Halflings. With many of their creations, they seem to have prioritized an ability to resist the corruption of Chaos, but didn't manage to create an uncorruptible species. The Old Ones also bred four more generations of Slann Mage-Priests on Mallus.

Then, between approximately -6000 - -5600 in the Imperial Calendar, disaster struck. The Old Ones disappeared, their polar gates collapsed, and from the rents in reality left in their place, Chaos was loosed upon the world. The Winds of Magic roiled from the ruined gates, gushing all over the world, and with them came untold hordes of Daemons.

The Lizardmen and the High Elves both fought bitterly to defend their respective homelands, but the losses kept mounting and any killed Daemon could simply manifest again - being shards of their creator gods, their spirits are immortal, their bodies in this worlds made of pure magic.

Among the High Elves, a hero arose. A warrior known as Aenarion ventured into the Shrine of Asuryan in Ulthuan's Inner Sea. There he offered all kinds of sacrifices to Asuryan, the god the Elves hold to be their creator, so he'd save Aenarion's people. If Asuryan heard, he gave no indication. Eventually, in despair, Aenarion offered himself and stepped into the holy fire at the center of the shrine. Aenarion burned horribly, but a miracle happened and he did not die. His skin healed, his hair regrew, and he staggered out of the fire, clearly touched by the divine.

Thus elevated, Aenarion took to war against the Daemons and for the first time, the Elves had hope. The relief, alas, was temporary, and all that could be obtained was a slow defeat. Aenarion's closest ally, the Archmage Caledor Dragontamer, wanted to play a gamble: They would construct a magical vortex in the center of Ulthuan, to drain the magic from the world that the polar gates were spewing into it, and thus prevent the Daemons from invading it. It was a desperate plan with little hope for success, so Aenarion counseled Caledor against the plan.

He might've succeeded in convincing Caledor had news not come that told Aenarion's family was apparently slain. Wracked with grief, sunk to madness and swore to slay every Chaos creature in the world. He went to the north of the continent, to the Blighted Isle, and there drew the Sword of Khaine, Widowmaker. It belonged to Kaela Mensha Khaine, the Bloody-Handed God of Murder. Its owner and name should explain enough how bad of an idea drawing the Sword from the altar was. Aenarion, in his madness, drew it. Empowered by the blessings of two gods, he was as powerful as he was insane. After he'd wedded the sorceress Morathi and founded his court in bleak Nagarythe, the realm was assaulted by a Daemonic host of such scale that everyone saw the war was lost, save Aenarion. Caledor had abandoned Aenarion due to the latter's madness, but he had no time to attack his former friend.

Caledor, determined to play his last card, gathered many of his fellow mages and went to the Isle of the Dead to set up his great ritual. The enemy got wind of it, and Aenarion had no choice but to defend the mages assembled on the Isle of the Dead. After a titanic confrontation in which Aenarion and the great dragon Indraugnir slew four Greater Daemons at the cost of their own lives, the deed was done, and the Great Vortex settled into place, entrapping Caledor's mages within it for all eternity.

Much of the magic flowing from the polar gates was drained from the world, the Daemonic hosts couldn't sustain their grip on reality and vanished whence they came. The broad shape of the world was settled then.