If I recall correctly, Rice describes revenants as vampires that become "brain damaged" (I am not sure if that was the term) due to them being bitten and entombed or buried and not being able to feed for a long time but I may have this wrong -- I definitely do not remember exactly what she said about how they came about.
This was intriguing but at least in Interview with the Vampire, it is mentioned only once. Did she explain further in other books? I know the word is not her invention, but does the idea of an insane vampire being called this occur elsewhere? I do recall that in the book I Am Legend, "insane" vampires were those that died and were reanimate rather than living people who had been infected.
Clarifications: I don't think Rice used the term "brain damaged" -- I am interested in more than one thing here:
- What term did she use instead of brain damaged?
- Does she explain revenants in later books?
- What causes the damage to vampires who become revenants?
- Do other writers than Rice and Matheson have "smart" and "dumb" vampires?