Apparently some criminal escaped from Riften . Surprise, surprise. We are to hunt them down and kill them, simple enough. The person you will hunt is only named "Criminal" and starts out just west of Riften. They will travel themselves though, so fast travel to Riften Stables and give chase! You should catch up with them pretty soon, and since they aren't particularly tough you will kill them in no time.
Note: This is a Radiant Quest , therefore, the criminal may start anywhere on the map. It doesn't make a lot of difference though. Simply fast travel to the nearest place and attempt to hunt them down to complete the quest.
Report your job complete back to one of the circle members for your reward of 300 Gold coins.