Cruciform

The Cruciform was something belonging to the Time Lords. The Dalek Emperor personally took control of the Cruciform during the Last Great Time War. The Master had been resurrected and was to protect the Cruciform but because he was so scared, he ran away and used a Chameleon Arch to turn himself into Professor Yana so that the Daleks couldn't find him. (DW: The Sound of Drums)

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, of course, means “cross-shaped”.

Behind the Scenes

The Crucible mentioned in the filming of The Stolen Earth has been revealed to be in no way connected to the cruciform.