Rauborn is a barman that works in the Ratpit Bar, found in the Rat Pits beneath Keldagrim for Overgrown cats. He sells stew for 20 coins and beer for 2 coins.
As he's located in the Rat Pits, you must have reached a certain stage during the quest Ratcatchers, more specifically in the part after you help Smokin' Joe, to talk to him.