I'd say the Barrow-Wights... Not much explaining to be done, the whole discription fo them unto me just seemed scary.
As all of you af course now they are based upon the Old Norse mythological creature's called "Draugar". (Singular = Draugr).
Origenally the Nazgul were barrow-wights as well, which would mean that the Nazgul are based upon Draugar as well, although there's not much to be seen back about that.