[Azred]

Jabidas is quite correct. A lich is what arises after a mage (not necessarily a necromancer) has prepared a container for his soul (called a phylactery), cast preparatory spells upon himself, and has concocted a potion of utmost lethality (using such tasty substances as wyvern poison, hemlock, etc.) The mage drinks the potion (which actually does kill him), lies dormant for three days (his soul has shifted to the phylactery), and if successful will arise as a lich; if not, then the mage is permanently dead.

There are both preistly and psionic version of liches, but the fundamental idea is still the same. The worst thing about a lich is that you never permanently kill them unless you also destroy the phylactery. All the liches you kill during the game will come back for you!