In the Tomb of Horrors module for Advanced Dungeons and Dragons, King Acererak, the mighty demi-lich is described like this:
"Ages past, a human magic-user/cleric of surpassing evil took the steps necessary to preserve his life force beyond the centuries he had already lived, and this creature became the lich, Acererak. Over the scores of years which followed, the lich dwelled with hordes of ghastly servants in the gloomy stone halls of the very hill where the Tomb is. Eventually even the undead life force of Acererak began to wane, so for the next 8 decades, the lich’s servants labored to create the Tomb of Horrors. Then Acererak destroyed all of his slaves and servitors, magically hid the entrance to his halls, and went to his final haunt, while his soul roamed strange planes unknown to even the wisest of sages. Joining the halves of the FIRST KEY calls his soul back to the Prime Material Plane, and use of the SECOND KEY alerts the now demi-lich that he must be prepared to do battle in order to survive yet more centuries."
All that now remains of Acererak are the dust of his bones and his skull resting in the far recesses of the vault. This bit is enough! If the treasure in the crypt is touched, the dust swirls into the air and forms a man-like shape…” and if enough attacks are made, “the dust will form into a ghost controlled by Acererak, and this thing will attack immediately.”
"If any character is so foolish as to touch the skull of the demi-lich, a terrible thing occurs. The skull of Acererak rises into the air upon the touch, and it slowly scans the party. There are 2 jewels set into the eyesockets and there are 6 pointed (marquis cut) diamonds set as teeth in the jaw. The demi-lich can tell which member of the party is the most powerful…"
"The soul of the strongest will be drawn instantly from his or her body and trapped within the right eye jewel, and the gem-eye will gleam with wickedly evil lights as the character’s body collapses in a mass of corruption and moulders in a single round - totally gone. The skull will then sink down again, sated. If it is struck or touched a second time, it will again rise and drain the soul of the next strongest character into its other eye. This process repeats through all 6 of the diamond teeth, and if the skull is still intact and still molested, it will pronounce a curse upon the remaining characters which will teleport them randomly in a 100-600 mile radius, each cursed to some [unthinkable] fate."
Pretty epic bad guy, but theoretically beatable. My guess is that he spent his time in ‘strange planes unknown to even the wisest of sages” working on his “jousting skills.”