the guild watchman is located at the magic vendor's store at the far left of Lumiest, and guards the staircase leading down into the Mages' Guild. If provoked, Lexus will attack with rapid dart spells (like those an exile would cast). If you enter the Mages Guild by teleporting Lexus away, killing him or through him being assassinated, you will be attacked by the guild members within, unless you use a disguise kit, or the incognito spell/scroll.