Cruciform

The Cruciform existed during the Last Great Time War. The Dalek Emperor took control of the Cruciform. 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.