When you return, go talk to Gabriella. She will give you a mission to kill Gaius Maro, the son of Commander Maro who would have helped you with the "Destroy the Brotherhood" quest if you had chosen that.
Travel to Dragon's Bridge and steal Gaius' travel schedule. After you've gotten it, you can read it to find out where Gaius is on any given day (check the date by hitting the Wait button). It is preferable to kill him in a city because you will get a bonus however he travels the roads alone so it will likely be easier to kill him while he travels. It is your call. Once you've killed him, plant the note on his body and return to the Sanctuary for your reward.