Phylacteries

Before the War
A phylactery is a vessel, often a glass vial, containing the essence of a magical being. The Circle of Magi and the Chantry used small phylacteries filled with blood—taken from apprentice magi—to track down mages that turn apostate, fleeing the wrath of the Templars. In Ferelden, some of these phylacteries were kept in the basement of the Circle Tower behind protective wards, but most were kept at an undisclosed location in Denerim. Phylacteries for first enchanters were stored in the White Spire, an Orlesian Circle and the Templar Order stronghold.

Phylacteries, 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 honing in on the fugitive by way of a "hot and cold" situation, i.e., the phylactery glows and the glow grows more intense the closer the Templar gets to the mage they are tracking. If the mage dies, the phylactery no longer glows.

Changes in Phylacteries and their uses since the inception of the War
The Libertarians still use phylacteries, since hunting maleficar is still a job that needs to be done. The difference being it is mages who use the phylacteries, not Templars.

The phylacteries of the First Enchanters in the White Spire in Val Royeaux were destroyed at the beginning of the War. Now, only those who are part of a Loyalist Circle have phylacteries there.

Phylacteries themselves have been upgraded. Those in the Libertarian Circles can only be used by mages. If a Templar was to get a hold of them they could not use them. No non magic user can.

Iago's "Theory of Enervation" regarding phylacteries
Iago has a theory about phylacteries. It has something to do with "living blood" and creating a source of blood power for blood mages that does not require a blood sacrifice.

"Why does dead blood not work? Because its DEAD! The source is dead. So its not the blood but the LIFE. Its life demons feed off... blood just contains life, or rather... it is the cord that connects magic to it..."

"So, if we can infuse spirit into dead blood... if we can breath life into enervated blood..."

"And thus Furia, you have no need for lyrium or messy sacrifice. No more using your own blood either, hahahahah!"

Phylactery Research
Phylacteries of Enervated Blood are currently being "investigated" in character.

Spirit infused enervated blood (phylactery with no living source) is being developed in Nueva Libertad.

This is a necromantic theory.