Lichdom

A lich is an undead spellcaster, usually a wizard or sorcerer but sometimes a cleric or other spellcaster, who has used its magical powers to unnaturally extend its life.
A lich is a gaunt and skeletal humanoid with withered flesh stretched tight across horribly visible bones. Its eyes have long ago been lost to decay, but bright pinpoints of crimson light burn on in the empty sockets.

Causes

An evil wizard may, by choice, extend their lives to continue their deeds. They become undead and a lich through this process.
An integral part of becoming a lich is creating a magic phylactery in which the character stores its life force. As a rule, the only way to get rid of a lich for sure is to destroy its phylactery. Unless its phylactery is located and destroyed, a lich reappears mere days after its apparent death.
The most common form of phylactery is a sealed metal box containing strips of parchment on which magical phrases have been transcribed. However, other forms of phylactery exist.

Type

Magical