The lines between death and undeath are not always as clear-cut as some would like. Revenants are those who have been “sent back” by some deity or other for a specific purpose. Unfortunately, that purpose is not always immediately clear. Also, the details of a revenant’s former life are also not always clear. They usually have an idea of what their old race used to be (though their features are much closer to those of Lord Voldemort from the Harry Potter movies), and can still take feats related to their old race.
Revenants are both living and undead, and as a result suffer effects that affect either type of creature (like a cleric’s Turn Undead burst power). On the flip side, though, it’s very hard to kill a Revenant… and even after they drop to 0 HP, they’ll still keep fighting (albeit in a dazed state) as long as they can keep making death saves, until they hit their negative bloodied value. That alone can make a revenant a terrifying foe. Equally scary is their ability to steal the death energy of a recent kill and use it to harm the next foe they hit.
Revenants are (thankfully) extremely rare, and don’t really have any societies of their own. Those that learn their purpose are usually singularly minded, and have been known to fade away into the rest of actual death once their purpose is fulfilled. Some legends tell of revenants who have regained full life upon completion of their tasks, but these are just stories, as far as anyone knows for sure.