Phylactery

A phylactery is a vessel, often a glass vial, containing the essence of a magical being. The Circle of Magi and the Chantry use small phylacteries filled with blood—taken from apprentice magi—to track down mages that turn apostate, fleeing the wrath of the templars. Some of these phylacteries are kept in the basement of the Circle Tower behind protective wards, but most are kept at an undisclosed location in Denerim.

Workings
Phylactery's, ironically are a form of blood magic. When a Templar wishes to track down a fugitive mage they will use the phylactery as a way of homing in of the fugitive by way of a "hot and cold" situation ie. the phylactery glows and the glow grows more intense the closer the Templar gets to the mage they are tracking.

Etymology
In many forms of fantasy fiction, a Phylactery is the vessel for the soul of an undead monster, particularly, a Liche.