After completing the last quest of the Thieves Guild, the Gray Fox will give you the promotion of Guild Master and the Nocturnal's Cowl. The Nocturnal's Cowl lets you see living things as energy even through walls. Note: When you equip Nocturnal's Cowl, you are the Gray Fox -- when guards see you, they will try to arrest you. Nocturnal's Cowl also lets you carry more items and increase your Sneaking skill.
When wearing the cowl, your bounty is 500 less whatever bounty your character already has and you are no longer who you are. You are the Gray Fox. Any felonies committed while wearing the cowl will be dismissed as soon as you take off the cowl.