A lich is a spellcaster who seeks to defy death by magical means.

Liches convert themselves into a skeletal undead creature by means of black magic and necromancy, storing their soul in a magical receptacle called a phylactery.

They do so to extend their life in order to study the deepest levels of magic.

With their soul bound to a material focus, they can never truly die.

If their body is destroyed, a lich can simply regenerate or find a new one in a relatively quick manner.

According myth, the only way to truly destroy a lich is to first destroy its phylactery, thereby removing their anchor to the material world, and then destroy its physical form.