Is there any information about the Rotting Man besides what can be found in the 3rd edition Unapproachable East? This sourcebook only says that he is a NE male half fiend human druid 12/Talontar Blightlord 10 and Chosen of Talona. There's nothing about his history, personality, looks, real name or motivations/goals (other than spread Talona's Blight everywhere).
Lost Empires of Faerūn has a little info on him as well, but I can't presently summarise those tidbits because I'm nowhere near that tome at the moment.