The Desolator

The Desolator was a planet-imploding bomb intended to be used if any sign of Cybermen were encountered.

History
The Desolator appeared to have been given to platoons in case of Cybermen encounters in the aftermath of the Cyber-Wars. If a Cyberman were to be spotted, the platoon's orders were to use the Desolator to implode the planet. The Desolator possessed a handheld detonator as well as a fallback voice-activation that only the platoon's captain or the Emperor could activate.

After the Cybermen were discovered on Hedgewick's World of Wonders, the Eleventh Doctor ordered the punishment platoon located there not to "blow up the planet," causing Captain Alice Ferrin and Porridge to explain what the Doctor meant to Clara Oswald. Clara took the detonator to prevent Ferrin from detonating the bomb, but she later attempted to do so using the fallback voice-activation. However, she was killed by a Cyberman before she could do so. The Cyber-Planner known as "Mr Clever" later destroyed the handheld detonator.

With millions of Cybermen awakening, the Doctor decided to use the Desolator to destroy the planet. However, without Ferrin or the detonator, this appeared to be impossible before Angie Maitland realized that Porridge was in fact the Emperor. Using his power as Emperor, Porridge was able to activate the Desolator which summoned the Imperial Flagship. After the Flagship transmatted everyone and the Doctor's TARDIS aboard, the Desolator destroyed the planet and all of the Cybermen upon it.

In the aftermath, Porridge had the Imperial Flagship search the wreckage for any sign of survivors. Finding no apparent signs, the Imperial Flagship left, missing that a single Cybermite survived the destruction of Hedgwick's World. (TV: Nightmare in Silver)