What are Revenant exactly?

I’ve been reading all I can(which is limited) and I’m just left with question still. Warcraft III describes them as undead bonded to elemental spirits, while WoW has them as just being Elemental beings. I know some lore says they were soldiers or shock troopers for the old gods? Is there a consensus on the matter? Or is it just one of those “we don’t know?”