The Crown of Sorcery, sometimes known as the Crown of Nagash, is a magical crown/circlet forged by the great Necromancer Nagash around -1300 IC in his fortress of Nagashizzar in Cripple Peak. The crown is infused with part of Nagash's malicious spirit, partially holding his spirit to the mortal world, and exerts a corrupting influence on anyone who wears it.
The Crown of Sorcery constantly whispers dark things into the minds of those who wear it; slowly and insidiously manipulating them, and bending them to it's will. The crown was made by Nagash to amplify his control of Dark Magic, or Dhar, in general, rather than the more specialised field of Necromancy. Though it is most powerful when worn by its creator, it greatly amplifies the magical ability of any wearer, and draws dark energies to them.
The Crown has had several owners over the course of history, extending its corrupting influence on all of them. The crown was first made by Nagash after his first defeat at the hands of the kings of Khemri and the various other cities of Nehekhara, who drove him out of that land. When Nagash invaded Nehekhara a second time, he wore the crown and it greatly amplified his magical abilities, allowing him to cast a great spell which laid the land low with disease.
King Alcadizaar, the last Nehekharan High King at Khemri, who was Imprisoned by Nagash, was freed by Skaven and given a sword of pure Warpstone with which to kill the Necromancer. Seizing his opportunity the king surprised Nagash when he was recovering from casting his spell and slew him, taking the crown he waded into the Mortis river which runs through Khemri and died from the wounds sustained in the great battle and the effects of his proximity to pure Warpstone (the sword).
Alcidzaar's body washed up on the bank of the Blind River in the Badlands and was discovered by Kadon, a young Human shaman. Kadon took the Crown, which was still gripped in Alcidzaar's cold dead fingers, and wore it. With the assistance of Ushoran, the first Strigoi Vampire, who had been drawn to the place by the Crown, he built the city of Mourkain and founded the civilisation of Strigos around it, which was obsessed with Necromancy -hence the Crown's influence.
After a time, Mourkain and Strygos were destroyed by a giant Waaagh! of Greenskins lead by the Orc Dork Redeye, who had previously been driven out the area by Strigos. Surviving Mourkain escape with the Crown. The Crown was eventually passed to Morath.
Morath was later defeated by Sigmar and Sigmar took the crown.
The next time the Crown surfaced was in many centuries later being worn by the Orc Warboss Azhag the Slaughterer who attacked The Empire and destroyed the province of Solland. After Azhag's defeat, the Crown was taken by the Grand Theoginist of Sigmar and is now located in the vaults beneath Altdorf, where it is locked away forever, to protect the world from it's evil.