Resar is Halruaan mage sent to retrieve secrets of skyship construction. Upon completion of the quest he will thank you and apologize that he must now kill you as you a possible "loose end" in his dealings with the skyship. Only Resar will attack you, which does not damage your reputation with the thieves guild (unless you cause collateral damage to any of the thieves while fighting Resar of course). If you've successfully completed the Narlen's Heist sidequest, Narlen will kill Resar when he threatens you.