Phylactery

A phylactery was the name given to the repository used to store the life force of lich. A mage had to create such a phylactery in order to become a lich.

Creation
Making a phylactery was a highly expensive and taxing effort, requiring a high degree of spellcasting ability and materials valued at 120,000 gp.

Powers
If a lich's physical body was destroyed, it could regenerate itself where its phylactery was located within a tenday. However, if the phylactery itself was destroyed, then the lich could not regenerate and would remain destroyed.

For this reason, liches sought to guard their phylacteries powerfully against any attack.

Description
Typically, a lich's phylactery was a sealed metal box inside of which were strips of parchment written with magical phrases. However, a phylactery could be any number of things, such as a ring or amulet.

Background
Phylactery, from Ancient Greek (via Latin) for "protectant", is a term for a charm or amulet, a Christian reliquary, or a translation of the Jewish tefillah. It is also the term for "speech scrolls" in Medieval art.

In D&D, the term is also used for various magical items worn on the head associated with morale and alignment.