The Sundering was the great civil war that fractured the Elven race into the High Elves and the Dark Elves, culminating in a magical cataclysm which sunk a large portion of Ulthuan -primarily sections of Tiranoc and Nagarythe (which is now known as the Shadowlands). In the timeline, the Sundering occured around –2700 IC.
According to birthright, Malekith, son of the first Phoenix King Aenarion and an Elven enchantress called Morathi, was the rightful successor to the throne of Ulthuan. But when Aenarion died, the ruling Council of Princes decided that Malekith was too impetuous and warlike, as Aenarion had led the Elves through a great war and the population was extremely weary of conflict. Declaring that Malekith was not fit to rule, they selected another to sit on the throne - Bel Shanaar of Tiranoc, a hero of the war.
Malekith was instead given the rank of High General and supreme commander of the armies. This suited him well, as he was an outstanding warrior and peerless general, just as his father was. During his time in the army he wandered the globe, and with his mother's guidance, started experimenting with Dark Magic. After some time, Malekith became discontent. In his mind the Elves must be a warlike people to face the ever-growing menace from the forces of Chaos. He became increasingly critical of the Princes, who grew soft and complacent under the protection of his armies. Worse yet, the Cult of Pleasure, a society of poets and artists, founded by Morathi herself, turned out to be a Cult of Slaanesh. Their practices were taking root in Elf society, being particularly strong in Nagarythe. Upon hearing of this, Malekith took personal command of the investigation and used this power to eliminate his political enemies, accusing them falsely of being cultists and executing them without trial.
The Civil WarEdit
It was then that Malekith saw his chance to seize his birthright. He claimed Bel-Shanaar was a worshipper of Slaanesh. Soon after the accusation, Bel-Shanaar died of poisoning. Those loyal to the Council accused Malekith of assassination. Those loyal to Malekith claimed Bel-Shanaar committed suicide out of shame. The argument between Malekith and the Council of Princes turned violent, and Malekith and his followers killed most of the Princes, claiming to save Ulthuan.
Now the Council was destroyed, Malekith had only to pass through the Sacred Fire of Asuryan, king of the Elven Gods. He stepped forwards, confident he would pass the ordeal just as his father did. Yet the Sacred Flame burned him, scarring him horrifically. His followers claimed treachery - that the fires must have been corrupted by magic to reject even the rightful heir. They fled back to the province of Nagarythe, and in confusion over the death of their king and council, the High Elves (comprised of the other Kingdoms of Ulthuan, and a small body of resistance in Nagarythe which would later become the Shadow Warriors) did not pursue.
Near death, Malekith was taken to one of his mother's temples. His body had healed sufficiently, but his mind was destroyed beyond repair and he had gone insane. He knew of the need to lead his followers into battle against the High Elves, but also knew his withered body would not survive the battle. Morathi called upon Hotek, a renegade priest of Vaul, to forge the Armour of Midnight, a suit of armour that would give life and strength to Malekith's flame-ravaged body. The armour fused directly to his skin, forming a sorcerous shell granting him immense supernatural powers. From that day forth, Malekith was known as the "Witch King", for he truly had become a figure of dread.
Once healed, Malekith was consumed with a bitter hatred for those who has resisted him . He summoned those Elves who were loyal to him to aid him in taking the throne by force. This included a large part of the standing military, most of his home province of Nagarythe and many other Elves throughout Ulthuan who believed in Malekith's cause.
Meanwhile, the High Elves started organising themselves and elected a new Phoenix King, Caledor the First. Caledor gathered his armies to him and a bloody civil war erupted between the High Elves and the followers of Malekith. Great victories were won on both sides, but in the end the scales tipped in the favour of the High Elves. Although their stronghold in Nagarythe was nearly impregnable, Malekith's followers were slowly outnumbered, as Caledor proved to be as equally skilled of a general as the Witch King. Finally, Malekith and Caledor faced each other directly at the head of their mighty armies. Caledor managed to defeat the Witch King, driving his army into the Marches of Maledor.
Desperate, Malekith decided on a final gamble: he gathered his sorceresses and attempted to unravel the Vortex of Ulthuan, a great swirling mass that prevented the return of the full force of Chaos. As Malekith and his coven began this dark ritual, the mages of Isle of the Dead intervened, weaving powerful counter spells. While the alternative would have been far worse (the complete unfettered spread of raw Chaos energy from the Realm of Chaos through the world, as it once had millennia before) the resulting magical backlash caused the cataclysm known as The Sundering, and submerged great parts of Nagarythe and Tiranoc beneath the Great Ocean.
As the Kingdom sank, several of the great fortress-cities of Nagarythe were pulled from the bedrock by sorceresses and kept afloat by their magic, though Anlec, the capital, could not be saved. Malekith and the surviving loyalists sailed their fortress-ships, which would become known as "Black Arks", to the New World and established a new kingdom - Naggaroth, in honor of their destroyed home.
As a result of the Sundering, and his defeat at the hands of Caledor, Malekith and his Dark Elves became obsessed with the death and enslavement of the High Elves, and the restoration of Malekith to the throne of the Phoenix King. They have launched a total of 5 invasions to date, attempting to claim the Phoenix throne for the Witch King, including the latest one in the Warhammer background and the current invasion in the Age of Reckoning in WAR. The Kingdom of Nagarythe has been is blasted and twisted to the point where it is unrecognisable as the majestic and beautiful land it once was. Rife with disruptive magical energies, it's shattered remains are now known as The Shadowlands and the descendants of those citizens of Nagarythe which resisted Malekith, known as the Shadow Warriors, now inhabit it. Most of the Kingdom of Tiranoc now lies underneath the Great Ocean of the World, including it's once-capital Tor Anroc.