The promotion to Prowler requires you to perform a single assignment, Ahdarji’s Heirloom, for S’krivva in Bravil. Before she will give you the assignment though, you must fence two hundred gold worth of stolen goods. S’krivva will ask you to steal back a ring that was stolen by an independent thief. You must find the ring’s owner, Ahdarji, in Leyawiin – you can find her at her house, Five Claws Lodge at lunch, or at the Three Sisters’ Inn in the evening. Find her and she will tell you that an Argonian named Amusei stole it from her.
Talk to the beggers in Leyawiin and they will tell you that Amusei is in the Leywiin Castle prison. Go to the Castle Dungeon and either bribe the jailor to let you visit Amusei or pickpocket the key off of him. Talk to Amusei and, if you give him a lockpick as a bribe, he will tell you that the ring originally belonged to the Countess of Leyawiin and she has now recovered it. Return and talk to Ahdarji and she will offer to pay you double to steal the ring.
Enter the Castle, go straight through to the Throne Room, and wait until 1:00 Am. Then, enter the Basement all the way to the right of the thrones and find the open-topped barrel against the south wall. The lever inside will open a secret passage through the torture chambers and into the Countess’ bedroom. Pick the lock on the door at the end, turn right, and pick the lock on the door at the end of this hall. Another lever will open the exit. Go up the stairs to the left and through the door. Avoid the guard, turn left, and break into the Countess’ Room. The jewelry box is on the table to the right of the bed. Break into it, steal the ring, escape the same way you entered, and sell the ring to Ahdarji. Return to S’krivva and receive your promotion. The achievement will unlock when the conversation with S’krivva is over.