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 the War Master 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)
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.