The Mask of Dark Earth is an ancient, evil artifact that represents the evil earth spirit and a villain from Sly 3: Honor Among Thieves.
The mask moves on its own accord, floating in mid air and possessing people to do its bidding. While under its control, the wearer becomes a constantly enraged giant. A particularly poignant example of this occurred when Carmelita Fox, pursuing the Cooper Gang to Australia, was possessed by the Mask and subsequently freed of its control by the selfsame gang. But unlike the mask's other victims (the miners), when it possessed Carmelita, Bentley tried to use his sleep darts to knock her out so they could take the mask off of her and hopefully destroy it. Unfortunately, it had the opposite effect. The chemicals in Bentley's sleep darts reacted to the mask and made her even bigger, growing to the height of 100 ft. The Mask of Dark Earth was destroyed at the end of the Cooper Gang's operation "Moon Crash", when Sly climbed up to her face to remove the mask, which returned Carmelita to normal. Carmelita's backup forces eliminated the evil artifact, but it killed the backup forces when it exploded.