The Black Baron's Mask is a treasure that can be obtained in Sly Cooper: Age of Thieves. It is found in the hidden vault in the episode Alice in Cooperland, and upgrades all of the playable characters, specifically Penelope.
"Back when Penelope was younger, she used to use the identity of the Black Baron, the ACES champion over in Holland. While the Baron's mask has definitely seen better days, it's at least good enough to wear again. It even prevents people from losing health by falling from dangerous heights, and gives Penelope extra defense from the guards' attacks."
This treasure is located in a vault hidden behind the White Queen's throne in the east section of Wonderland. The three passwords needed to open the vault are "Forgive", "Change", and "Sorrow", and the numbered code needed to access the vault is "5-4-7-7-7".