Skovox-Helicon war

The Skovox-Helicon war was an interplanetary war between the Skovox, a highly advanced and belligerent species, and their arch rivals, the Helicon, a highly advanced and easily annoyed species.

The War
The Skovox and the Helicon preferred to conduct their battles by proxy, using robots on innocent planets located between their two star systems, which would end up as radioactive wastelands or previously uncharted asteroid fields. The Skovox created the Skovox Blitzers with the intention that they would deliver a decisive victory. They proved successful against the Helicon, homing in on their tell-tale particles, until the Helicon found a way to convince the Blitzers that they were their superior officers and that they should attack the Skovox, homing in on their tell-tale Artron emissions. Unfortunately, they neglected to use the word "instead" in their instruction, and the Blitzers turned their fire on both the Skovox and the Helicon.

During the resulting chaos, all the Skovox who knew the shutdown codes for the Blitzers were lost, and whenever the Skovox or the Helicon tried to command the Blitzers to shut down they self-destructed with catastrophic consequences, as each carried enough explosive to destroy a planet. The final result of the conflict was a large empty patch in the galaxy where their two solar systems used to be. Most of the Blitzers were destroyed but a few survived, awaiting new orders whilst drifting through space. They continued to scan for Helicon and Artron emissions in the hope of finding new targets. The Skovox-Helicon war was covered by historians who had been allowed access to the "Monster Vaults" of the databanks in the Doctor's TARDIS, whom reported the belief that there remained over 800 Blitzers still fighting the conflict. (PROSE: )

Aftermath
A Skovox Blitzer came to Earth in the early 21st century, attracted to the area around Coal Hill School by the high number of artron energy emissions. Whilst there, it killed Matthew, a local police officer. The Twelfth Doctor detected the Blitzer and trapped it in Coal Hill's school hall and projected it into the Time Vortex using chronodyne generators. However, Danny Pink had inadvertently interfered with the generators and the Blitzer was not projected as far into the future as the Doctor had planned, reappearing just two days later rather than a billion years. Before it reappeared, the Doctor developed a device that would make his orders sound as though they were coming from one of the robot's superiors. With the assistance of Clara Oswald and Danny, he again trapped it, and ordered it to deactivate. He then left it to self-destruct in space near the Olveron Cluster, telling it to "enjoy [its] war". (TV: )