Envy demons are a new type of demon appearing in Dragon Age: Inquisition. The Templars know of them, but these demons have rarely been encountered. Their nature is to seek out a host whom holds a position of leadership, then possess it, assuming the host's identity and physical form after learning its secrets and desires. Afterward, it becomes next to impossible to recognize the person from the demon, whose then a perfect replica of its host, a foolproof impostor, that no one under its covert control would dare question.
The Lord Seeker Lucius Corin makes use of a Envy demon pawn to masquerade as him and lead the Templar Order while he led the Seekers of Truth to their doom, whom he discovered were immune to Corypheus's control despite being fed Red Lyrium. The Demon appeared in Val Royeaux to officially announce that the Chantry no longer held any control over the Templars and leaves after proposing an alliance to the Inquisitor
If the Templars are recruited into the Inquisition...
Should the Inquisitor takes 'Lucius' up on it's offer it is revealed that the entire meeting was a elaborate trap: to purge the order of the low few Templars not affected by the Red Lyrium and kill and replace the Inquisitor. Towards this end the Demon begins gathering information on the Inquisitor by sending it's subordinates to their death's against the them in order to gather information on their leadership style.
Following this the Demon drags the Inquisitor into the fade where it attempts to further goad them in order to learn more information on the Inquisitor by killing illusions of their allies as well as taunting them with images of what the Inquisition will look like under it's rule. The Inquisitor is aided by a Compassion Spirit named Cole who guides him in purging his body of the Demon.Following this the Inquisitor and the uncorrupted Templars team up to fight Envy's Red Templars while trapped in a barrier set up by the demon. After the barrier is dispelled The Inquisitor and their group confront the Envy Demon in the courtyard aided by Cole, whom partway through the battle assumed The Inquisitor's form to fight the group. Once the demon is finally dead the remaining uncorrupted Templars are either conscripted or made the Inquisitions allies.
If the Mages are recruited into the Inquisition...
Should the Inquisitor not side with the Templars the fake Lucius Corin is instead revealed as a Demon later on in Cassandra's personal quest and that the fate of the Envy demon is unknown.