Seven Princesses of the Tombcastle (墓城七姫 Bojō Nanahime) is a group of seven vampiric Nosferatu sisters.
Z/X Encyclopedia Entry
Head of distinguished Nosferatu clan, who keep refusing the intervention of the Knights of the Round Table who was endowed with overwhelming authority in the Black World. The seven beautiful sisters—from the first princess, Asia to the seventh princess, Brutia—are said to living harmoniously at the Tombcastle in remote area, but no one who went to confirm it has returned.
Their true form are what one would call as vampire. In contrast with Diabolus that were sucking the lifespan, they achieved perpetual youth and immortality by sucking the lifeblood of living things.
By the way, they are also currently being served by the Three Stygian Maids, who has devoted themselves to a multitude of masters.