Dr Randulf Harlow is a retired vampire slayer in the Blue Moon Inn in Varrock. If a player has started Vampire Slayer, they must buy him a beer in order to convince him to teach them how to kill Count Draynor. Dr Harlow will then give players a stake. After completing the quest, Dr Harlow will go back to what he was saying before ("Buy me a beer pleash").
His new look resembles Abraham Van Helsing, the protagonist and vampire hunter from Bram Stoker's Dracula. It is also likely that he was based off the character originally.
He could also be based on psychologist Harry Harlow, who was known to be an alcoholic.
At the mention of "How to kill a vampire", Dr. Harlow resumes a normal, sober state. His speech isn't slurred, and it seems the only thing that could snap him out of his inebriated state.