Revenant

"An entire unit of men, all slain by one creature. I didn't believe it at first, your Perfection, but it appears that this is so. We have a survivor, and while at first I thought his rantings pure exaggeration... it appears to be no simple skeleton. The descriptions of the creature's abilities were eerily similar to those our brothers at Val Fermin encountered almost an age ago: men pulled through the air to skewer themselves on the creature's blade, and attacks so quick that it was able to assault multiple opponents at once. No, your Perfection, what we have I have decided to dub a revenant and nothing less."
― From a letter to Divine Amara III, supposedly

A revenant is a creature that appears in literature. In these bodies of fiction, it appears as a powerful ceaseless. They have access to magic, usually of utilitarian nature, which assist its primarily-martial fighting style.
Stories tell of revenants existing, but little proof has surfaced to show them as any more than just boogie men.

Summary

Revenants are supposedly formed when someone dies and their lifeless body is possessed by a desire demon, creating a ceaseless. It's believed that one can arise if someone dies in such a substantial amount of grief or anger that is hard to fathom. It would seem as though even if one died vehemently hating all of Thedas, that may not be enough to create a revenant. More raw emotion would be required.
From this transformation, the ceaseless would form its own armored carapace through the strange energies of the Fade and unsheath a sword that stings with cold, lifeless energy. Depictions have shown revenants to able to use telekinetic powers, conjure walls of weapons, alter gravity around them, and bathe themselves in necrotic forces as though surrounded by shields.
There is one hope for the common people of Thedas, however! Bardic tales surrounding the creatures seem to suggest two methods when confronted with such a powerful creature. The first is to run for miles and miles as the creature spoils your harvests, burns your village to ash, and corrupts anyone they can get their hands on. Telling a Templar is much better than doing just about anything else.
If one is feeling brave, however: the other solution would be fire! It works because in the moment that the flame touches the revenants skin, the flames ignite into the same flames that burned Andraste alive. The Maker's holy fire is completely outmatches such a foul monstrosity. This is a dangerous approach since if the revenant catches you before you catch it, you probably don't stand a chance. This will only temporarily hinder it, however, because demons are far trickier. To beat the demon at its own game, quickly uncork a vial of recently drawn water. If you notice the water's color go from clear to black, you've captured that demon in its new prison.