Cruciform

The Cruciform was a location (PROSE: The Whoniverse) that existed during the Last Great Time War. It was kept hidden by the Time Lords, though eventually it was discovered by the Dalek Empire, leading to a battle for control over it.

In a previous timeline, this battle turned out in the favour of the Time Lords, who were able to prevent the Daleks from taking the Cruciform, but after used the Heavenly Paradigm to alter the timeline, an unforeseen consequence of his meddling lead to the outcome of the battle being a Dalek victory, allowing the Dalek Emperor to take control of the Cruciform. (AUDIO: The Heavenly Paradigm) The Master, horrified by his actions, ran away and used a Chameleon Arch to turn himself into a human infant and hide near the end of the universe. (TV: The Sound of Drums; AUDIO: The Heavenly Paradigm)

After taking the Cruciform, the Daleks brought the Time War directly to Gallifrey. (PROSE: Dalek Combat Training Manual) The Tenth Doctor said the Cruciform fell at the same time that Gallifrey was burned. (COMIC: The Forgotten)

Other mentions
, while pretending to be the Doctor, stopped the Dalek Emperor at the Cruciform Anew. (PROSE: Anything You Can Do)

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

In writing Doctor Who: The Encyclopedia, Gary Russell interpreted from the reference in The Sound of Drums that the Cruciform was situated on the Time Lords' homeworld of Gallifrey. Episode writer Russell T Davies, who would proofread The Encyclopedia, was clear in his own mind that it was not, that the Cruciform was "more terrible than I can say, of a scale and ferocity that's absolutely untransmittable on prime-time," a larger part of the "disgusting, horrific, temporal hell" that was the Time War. (DWM 388)

The Whoniverse would later state that the Master's encounter with the Dalek Emperor was "at" the Cruciform, confirming that it was some kind of location. (PROSE: The Whoniverse)