"Lich" is an old Anglo-Saxon word meaning "corpse," and the transition to a gaming creature seems to have been through the writings of Clark Ashton Smith and other pulp scriveners who resurrected the hoary term to describe the sorcerous, shriveled walking dead.
How does the lich get that way? Following the idea of the lich as magical cyborg, it endures a seven-step ritual of replacing the parts of its spirit and soul with sorcery. It's possible to meet with a wizard at one stage or another of this transformation, a true fractional lich (as opposed to the curiously named demilich, actually more powerful than a full lich). These horrible rituals include self-mutilation, trepanation, and worse...
1. The wizard gives up the eyes, the windows of the soul, replacing them with icy burning sockets having full-spectrum vision to 120'.
2. The wizard gives up the breath, the door of the soul, replacing it with a magical voice that causes fear as a full-blown lich does.
3. The wizard gives up the brain lobe of Memory, the treasury of the soul, replacing it with a gem containing an intangible and infinitesimally compact library of lore. This ensures that the lich will retain super-genius intelligence and spell knowledge without the need to consult books.
4. The wizard gives up the brain lobe of Reason, the throne of the soul, replacing it with another gem containing a distillation of the wizard's own methods and rationality. This gives absolute self-control and resistance to enfeeblement, polymorph, and insanity.
5. The wizard gives up the brain lobe of Instinct, the foundation of the soul, sacrificing it utterly. This gives resistance to charm and sleep, and indeed freedom from all desires save those for knowledge, power, and the suffering of others.
6.The wizard gives up his or her soul, replacing positive energy with chill negative, and gaining the paralyzing touch ability, death magic immunity, or possibly even more dreadful powers of the Negative Planes.
7. Finally, the wizard surrenders his or her life, and the body begins to decay, replacing magic-user levels, hit points, and other stats with monster hit dice and stats, and acquiring the remaining resistances.
A contradiction in the lich-as-written may also contain the key to turning it. How does the lich's pride and vanity square with its appearance as a moldy old skeleton clad in rags? Surely if it is motivated by pride, the lich will find some way to keep up appearances. For example:
1. The lich casts a continual illusion, appearing as an attractive ideal of the being it was (or never was) in life.
2. The lich wears an iron mask, armor of fearsome construction, and mailed gauntlets to hide its skeletal state.
3. The lich has embalmed its bones in a waxen compound, creating an eerily lifeless similitude of dead-eyed face and cold hands.
4. The lich has opted for skeletal glory, studding its bones with rich metals, enamels and gems.
5. The lich appears as a skeletal horror, but has cast an illusion on itself, so that it sees itself as fair and youthful, its dank surroundings as a pleasure garden.
6. The lich has simply subjected itself to permanent invisibility.
Turning the lich consists, first of piercing or dispelling the illusion, then presenting the sacred symbol while subjecting it to mockery, to dethrone its sin of pride. Difficult, yes, but you didn't think you'd get away with just a d20 roll to fend off the king of the undead?