Raynil Dralas is a dunmer and reported to be a vampire hunter who is currently working the city of Bruma. He has a room at Olav's Tap and Tack. He wears light armor. When you enter the tavern you will hear several people discussing Raynil and also Bradon Lirrian. This will update your speech requests. Ask about Raynil the next time you speak to someone and you'll be told about Bradon being a vampire and Raynil dispatching him. Once at Bradon's home speak to his widow and to the investigator to obtain the A Brotherhood Betrayed quest.
During the quest you will not actually meet Raynil until the very end. For the conclusion of the quest you must chase him to Boreal Stone Caves, however, you only have a limited amount of time to do this or else he will escape and you will not be able to finish the quest. When you have dealt with him inside the cave, make sure you retrieve all the keys from his body as they are required to open the chest of loot in the cave.