Armageddon Sapphire

In a parallel universe, the Armageddon Sapphire was a superweapon created by Rassilon, born from his darkest moments and most terrible fears, and stored in the Slaughterhouse on Gallifrey. The Sapphire was the darkest of the Time Lords' dark secrets, buried so deeply inside the Matrix that it had taken technicians a thousand years to prove its existence and discern its function and purpose.

It combined everything Rassilon had learnt about the fundamental nature of reality into a single device, more terrible than Time Destructors, particle disseminators, or the De-mat Gun. The Armageddon Sapphire simply tore reality apart, agitating Higgs bosons to the point where the space-time continuum collapsed into Calabi-Yau Space. Theoretically, the universe could eventually re-form over billions upon billions of years, but it would bear no resemblance to what had gone before. Once it was activated, it could not be stopped.

In the parallel universe created by the Quantum Archangel, becoming darker and corrupted by the Chronovores as they fed on it, the Time Lords were at war with the Enemy and the Sixth Doctor, as Lord President Admiral, ordered the Sapphire slaved to his TARDIS, choosing to destroy the universe than let the Enemy rule it. Before he could activate it, Paul Kairos stopped him and returned him to the main reality, along with the Master who'd been helping the Enemy. (PROSE: The Quantum Archangel)
 * Whether or not the Armageddon Sapphire existed in the main universe was never clarified.