Blue Moon Rune

"The Blue Moon Rune is one of the 27 True Runes, symbolizing cruelty and compassion. The Blue Moon Rune turns its bearer into a vampire and also gives the bearer the ability to turn others into vampires. At first, the bearer of the rune will be overwhelmed by bloodlust, making them hunger for the forbidden human life force. However, as their mastery over the rune increases, the barer is able to tap into the powers of compassion and are able to live on without feeding on blood--- a benefit shared by those vampires sired by the Rune's power, provided they stay within the same area as the rune itself. Once torn from it, however, they would revert to bloodlust and would need to drink the blood of other living creatures or perish. In addition, vampires who hold the Blue Moon Rune are more resilient against normal vampire weaknesses such as sunlight or holy weapons, and have extra abilities, such as the capability to transform into a bat at will. Only the bearer of the Blue Moon Rune has the ability to remain ageless, a trait shared among all True Rune bearers.

Spells of the Blue Moon Rune usually come at a cost. Commonly noted is the Blue Moon Curse, in which 100 sacrificed souls are used to strike down a foe."

Past bearers: Sierra Mikain, Neclord
Current bearer: Sierra Mikain

Information taken from [link] (because they can describe it much better than me!)

Blue Moon Rune belongs to Konami (Please make Suikoden VI!)