A revenant is, according to Wikipedia:
...a visible ghost or animated corpse that was believed to return from the grave to terrorize the living. The word "revenant" is derived from the Latin word, revenans, "returning", from the verb "revenire"; in French, "revenant" means "returning".
These revenants are restored from the dead in their former body. Being restored, however, does not heal wounds but the wounds are no longer fatal. In fact the body will remain reanimated and aware until the following sunrise when it will collapse to ash unless it kills or causes to die someone else.
Once they do that, they buy one additional day. Only one per day. After sunrise passes they must kill another person. No matter how many people they kill in a single day they only ever earn the next day.
While a revenant they cannot be killed. They can, however, expire. If prevented from killing by incarceration, incapacitation, or whatever (dismemberment, burning, etc.) then the following sunrise they will crumble to dust.
Revenants have no powers that they didn't have in life. They are no stronger, faster, smarter (well other than a knowledge of the reality of the supernatural, perhaps). The only difference between a revenant and a human is they can't be killed.