Skovox Blitzer

A Skovox Blitzer was an alien robot built for war, said to be among the most dangerous weapons ever created. A single Blitzer carried sufficient explosive power to destroy an entire planet.

Characteristics
The robot's physical form is composed of a humanoid torso with an alien-shaped head and stocky arms on top of a larger box-like structure (which presumably houses the power source / explosive material), with four crustacean-like legs in the corners that propelled the robot when on the ground to about the same speed as a running human. According to the Twelfth Doctor, the Blitzer can fly as well as travel across space, hinting that it had homed in on the target area on Earth without external help. This would also account for its strange, floating method of movement when on the ground, with the legs being responsible for propulsion without being slowed down by supporting the bodies weight as well. Its weapons are built into the arms with the right arm possessing a triple-barreled rapid fire weapon, which shoots three simultaneous green energy bolts that cause its target to rapidly burn and disintegrate, and the left arm housing a single barreled energy weapon that fires white energy bolts that can explosively vapourise anything within about two meters of the impact point.

Unusually, they were designed to vocally state aloud whatever they were mentally processing, from environmental information that they had scanned to whatever instruction they were currently following, possibly to intimidate the intended target. They were also programmed to think that the answer for almost every problem was to destroy the source/obstruction and apparently they had been designed to not differentiate between enemy combatants and unarmed civilians despite any claims from the target of being friendly and harmless. If ordered to shut down without the necessary final input code, they would self-destruct and destroy the planet that they were on. This was probably intended as a deterrent to any attempts by the enemy to take command of the Blitzer by mimicking its superiors.

History
One came to Earth in the early 21st century, attracted to the area around Coal Hill School by the high number of artron energy emissions. It was detected by the Doctor, who trapped it in the school hall and projected it into the Time Vortex using chronodyne generators. Whilst there, it killed Matthew, a local police officer. Matthew's consciousness was one of several victims who were transfered to the Nethersphere.

Danny Pink had inadvertently turned off some of 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, he 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 and told it to enjoy its war. (TV: The Caretaker)

After the Twelfth Doctor's mind was connected to a mind scythe, the "holiday snaps" the Doctor showed Kygon Brox included a Skovox Blitzer. (COMIC: The Instruments of War)