A Revenant is always a Demon and can look like most any real or mythical being. It was first documented in the city of Thebes, formerly Wasat, later stories were told that it came to Earth from another world, summoned here by Sorcery. The actual form is as varied as pigs in a poke, but they all are demons. You know what those are, right. I'll spell that big word out for one of you. D E M O N S Can you count that out, slowly now, one more time.
Seriously, all that is right there in books,(If you read Sumerian) or on the Internet, type R E V E N A N T(One word, no Spaces) in your search engine. You will find that outside of actually facing one, in the flesh, they can take many forms and majority of pain and suffering will be in your mind. The higher the IQ the more damage. One of you need not worry, IQ needs to be 32 or over so your safe, for now.
Thanks for reading, by the way, one of those poets was S. T. Coleridge, but Hell, that was long after Rasney.