Lord Ruthven is the name of the title character in Dr. John Polidori's work The Vampyre, which in turn inspired multiple operas and plays. Polidori tells of Ruthven's friendship with a young man named Aubrey who learns the vampire's secret but is honor-bound by an oath not to reveal it, even as Ruthven marries (then kills) Aubrey's sister. At this point Lord Ruthven vanishes. Other accounts (such as Bride of the Isles, transposing the tale to Scotland) may chronicle his destruction (usually based on a failure to fullfill his pact with Satan).
Ruthven is apparently an atypical vampire in some ways. Like Vampirella he suffers no harm from sunlight. More, although he can be harmed by ordinary weapons the rays of a full moon completely restore him.
Philip José Farmer mentions the family of Lord Ruthven in his book Tarzan Alive. Charles Loridans' work "Children of the Night" (see below) also refers to an important vampiric occut tome called the Ruthvenian.