The Order of the Virtuous Blood is an order of citizens located in the Imperial City Temple District dedicated to hunting down vampires and slaying them.
At the beginning of the quest of the same name as the Order, Seridur, is the head of the order and the order's base is in the basement of Seridur's House. Depending on what you do during the quest, Roland Jenseric can become the Head of the Order at which point he will pay you 250 gold for each Vampire Dust you bring him.
The original head, Seridur, was a vampire so their vampire hunting is questionable. They claim to be looking for vampires in Imperial City but haven't found out about any vampire activity going on their.
Their is only one rank in the order and theirs only one quest. The player can become an Honorary Member of the Order (brother or sister depending on gender). To get membership you must complete the quest and kill Seridur at which point Roland Jenseric will offer you membership into the order. The only true benefit is the 250 gold you get from giving him vampire dust.