A powerful spellcaster willingly transformed into undead, soul stored in an external location
A lich converts itself into a skeletal creature by means of necromancy, storing its soul in a magical receptacle called a phylactery. In some sources the method of becoming a lich is referred to as the Ritual of Endless Night. The lich creation process is often described as requiring the creation and consumption of a deadly potion which is to be drunk on a full moon; although the exact details of the potion are described differently in various sources, the creation of the potion almost universally entails acts of utter evil, such as using as an ingredient the blood of an infant slain by the potential lich's own hand, or other, similarly vile components. The potion invariably kills the drinker but if the process is successful he rises again some days later as a lich. Occasionally, this metamorphosis occurs by accident as a result of life-prolonging magic.
Unlike most other forms of undead creatures, the lich retains all of the memories, personality, and abilities that it possessed in life — but it has a virtual eternity to hone its skills and inevitably becomes very powerful. Like other powerful forms of undead (such as a vampire or mummy), a lich has unnatural powers owing to his state. For example, he can put mortals in a paralyzed state of hibernation with a touch, making them seem dead to others, and can, through his typically powerful magical spells, summon other lesser undead to protect him. Liches can radiate an aura of horror which can send weak-willed would-be foes to flight. The lich is capable of sustaining tremendous physical damage, and is immune to disease, poison, fatigue and other effects that affect only the living. However, despite all his undead "gifts", a lich's most valuable resources are his vast intellect, his supreme mastery of sorcery and limitless time to research, plot and scheme.
Since a lich's soul is mystically tied to its phylactery, destroying its body will not kill it. Rather, its soul will return to the phylactery, and its body will be recreated by the power keeping it immortal. Thus the only way to permanently destroy a lich is to destroy the phylactery as well. Therefore, the lich will generally be extremely protective of the priceless item. The phylactery, which can be of virtually any form (the default form is a metal box filled with rune-covered papers, but it usually appears as a valuable amulet or gemstone), will often be hidden in a secret place and protected by powerful spells, charms, monsters and/or other servants; the phylactery itself is usually of magical nature, meaning its destruction will generally be little easier than attaining it.