In a continuing public service, we now present a brief entry from the Encyclopedia Ardrisia:
The Lich (pronounced LITCH) is the most feared, most dangerous of all the undead. Whereas a vampire comes about by being bitten by another vampire (which begs the question, which came first, the vampire or the bite?) and zombies come about as the result of a magical plague, the lich is someone who chose to become undead. Usually powerful wizards or clerics who have conducted macabre rituals to extend their lives, lichs are terrible and deadly wielders of magic, extraordinarily difficult to defeat, and have lifetimes of experience at their disposal. The only way to reliably destroy a lich is to destroy its phylactery, a receptacle used for housing what is left of its soul. Without doing that, a lich could be reduced to bone splinters and it could still rise again, constructing a new body in a short period of time using its fell arts.
There are rumors that another sort of lich exists called the psuedo-lich. While these rumors have not been confirmed, it has been theorized that a psuedo-lich would possess the practical immortality of a lich while not having any of its magical might, instead possessing one or two minor magical abilities. Such extreme measures to stave off death would only be taken by the truly foolish, desperate, or both.
The Encyclopedia Ardrisia , volume 17 (Lakelands to Mumblefleas), Kilanio University Press, 48th edition.