Seven Kingdoms: Seowyn's Crossing
A lich is a powerful Undead magic wielder, considered one of the most dangerous types of Undead that there can be. Powerful priests or mages, if they value immortality over morality, can perform a terrible ritual that gives them a prolonged life beyond death. Their flesh withers, and their spirit endures within their remains.
All liches have a phylactery in which they store their life essence. If a lich is slain, its spirit returns to its phylactery to reform. A lich can only truly be destroyed if its phylactery is destroyed as well.
Eventually, even their animating spirits cannot maintain a lich’s form. they decay and disintegrate, becoming little more than dust and some bones. These remnants, no longer constrained by even that much connection to their life, become demi-liches, which are considered even more terrible.