Thousands of years ago, the High Elves were a unified race living in the island kingdom of Ulthuan.
The first Phoenix King, Aenarion the Defender, was the greatest warrior to walk the earth. It was by his hand that the first great invasion of Chaos was thwarted. Aenarion was married to Astariele the Everqueen who bore him two children, Yvraine and Morelion. During the war Astarielle was murdered and Oakheart the Treekin took the children and hid them safe within the Gaen Vale. Thinking them dead, Aenarion in a bloodthirsty rage, drew the Sword of Khaine from its resting place and waged a new war of vengeance on the forces of Chaos. During the next war, he rescued an Elven seeress from a Slaanesh warband. Her name was Morathi, and she was beautiful beyond description. Aenarion fell deeply in love with her, and she bore his son Malekith. Under the tutelage of both his father and mother, Malekith became a formidable warrior, a skillful general and an accomplished sorcerer.
According to birthright, Malekith was the rightful successor to the throne of Ulthuan. But when Aenarion died, everybody gathered in the Glade of Eternity to decide who the next Phoenix King shall be. It turned out that Yvraine was now the Everqueen and she had decided to marry Bel Shanaar of Tiranoc, a hero of the Daemon Wars, and he would be taking the throne. Morathi was outraged, yet she soon found they had tried to make her part of this decision, but she had had no part of it. Malekith was the first to accept the successor and bent his knee to the new king, silencing a major outcry from his people and mother in support of him.
Malekith was relegated to the rank of High General and supreme commander of the armies. This suited him well, as he was an outstanding warrior and peerless general in the manner of his father. During his time in the army he wandered the globe, and with his mother's guidance, started experimenting with Dark Magic. After a few years Malekith became discontented. 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 nobles, 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; had become a front for a Chaos cult of Slaanesh, was taking root in Elf society, being particularly strong in his homeland of Nagarythe. 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.
Then events turned critical. Malekith claimed Phoenix King Bel-Shanaar was a worshipper of Slaanesh and Bel-Shanaar died of poisoning shortly afterwards. Those loyal to the Council accused Malekith of assassination. Those loyal to Malekith claimed Bel-Shanaar committed suicide because he couldn't live with his failures. The argument between Malekith and the Council of Princes turned violent and Malekith and his followers ended up killing most of the Princes in the name of saving Ulthuan (or to simply destroy all opposition to his ascension; both sides say different).
Now the Council was out of the way, there was only one thing left to do in order to become Phoenix King. Malekith had to pass through the Sacred Fire of Asuryan, king of the Elven gods. Malekith stepped forwards, confident he would pass the ordeal just as his father had done. Yet the Sacred Flame would not suffer Malekith's polluted body and it burned him, scarring him horrifically. Malekith's followers claimed treachery - that the Sacred Fires of the Creator God Asuryan must have been corrupted to reject even the rightful heir. They fled back to the province of Nagarythe with their leader. In confusion over the death of their king and council, the High Elves did not pursue.
Near to death, Malekith was taken to one of his mother's temples. His body had healed sufficiently, but, according to the High Elves, his mind was destroyed beyond repair and Malekith was completely lost to insanity. 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 rigours of battle. To that end, Morathi commissioned Hotek, a renegade priest of Vaul, the Elven smith god, to forge a suit of armour that would give life and strength to Malekith's flame-ravaged body. Malekith had the armour, still white-hot from the forge, 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 had 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 the kingdom 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 becoming outnumbered, and Caledor was proving himself to be as skilled a general as Malekith himself. Finally Malekith and Caledor faced each other directly at the head of their mightiest armies. Finally 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 that prevented the return of the full force of Chaos. As Malekith and his coven began this dark ritual, the mages of the Isle of the Dead intervened, weaving powerful counter spells. The resulting magical backlash caused the cataclysm known as The Sundering, and submerged great parts of Nagarythe and Tiranoc beneath the sea.
As the province sank, several of the great fortress-cities of Nagarythe were pulled from the bedrock by sorceresses and kept afloat by their magic. Malekith and the surviving loyalists sailed their fortress-ships, which would become known as "Black Arks", to the continent called the new world and established a new kingdom that they named Naggaroth in honor of their destroyed home.
As a result of the Sundering, the overriding obsession of Malekith (and by proxy much of the Dark Elf society) is the death or subjugation of the High Elves of Ulthuan and the restoration of Malekith to the throne of the Phoenix King.