The Overlord was a hard-core, major sorcerous villain. He was so hard-core that when he decided to make a sorcerous artifact as a focal point for his evil spells, everything he tried - dragon teeth, Viking bones, dentist drills - would simply shatter under the stress of the titanic unworldly forces the Overlord was wielding to sorcerously smite his foes. Eventually, the Overlord decided that only he himself possessed the power to channel and focus these unholy energies. Consequently he ripped his own skull right out of his head in order to fashion the deadliest magical artifact ever created.
Nobody knows how he did it. There was only one witness to the procedure, the Overlord’s chief sorcerer Ulnar Bentwand, and Ulnar spent the following week vomiting continuously until he managed to crawl to the edge of a lava pit and throw himself in. Others have tried to make their own weaponized skulls, but few are willing to use their own, and nobody has managed to produce any magical artifacts that are remotely as powerful as the Mageskull. (Although for a while the market was positively awash in enchanted skulls, whose magical effects range from "sort of dangerous" to "amusing lava lamp alternative.")
The Overlord is gone, but the Mageskull remains: mighty, mysterious, and with more than a touch of the ick.