Cruciform

The Cruciform existed during the Last Great Time War. It was kept hidden by the Time Lords. After the Master used the Heavenly Paradigm, the Dalek Emperor took control of the Cruciform. (AUDIO: The Heavenly Paradigm) The resurrected Master was so frightened, he ran away and used a Chameleon Arch to turn himself into Professor Yana and hide near the end of the universe. (TV: The Sound of Drums)

The Tenth Doctor said the Cruciform fell at the same time that Gallifrey was burned. (COMIC: The Forgotten)

Behind the scenes
The Cruciform hasn't been confirmed to be either a location, a weapon, a ship or some kind of object. It has only been mentioned, not described. The word "cruciform" means “cross-shaped”, and is, amongst other things, a type of real-life screwdriver.