Twelfth Doctor (The Curse of Fatal Death)

The Twelfth Doctor was a possible future incarnation of the Eighth Doctor.

Biography
This incarnation came into being when his previous incarnation regenerated after an electrical mishap.

The twelfth incarnation noted that he had wasted three incarnations in a very short time span simply because he "forgot to unplug". He proved the shortest-lived incarnation, only living long enough to shut off a zectronic energy generator before being hit by excess zectronic energy, subsequently dying due to the exposure. Allegedly, zectronic energy would kill Time Lords beyond any hope of regeneration. He gave a final speech to Emma and the Master, asking them to "look after the universe" for him, as he had "put a lot of work into it". He promised to explain later before dying. His foes, the Daleks and the Master, vowed to reform and live better lives because of him.

Unexpectedly, he regenerated into a new incarnation. (DW: The Curse of Fatal Death)

Behind the scenes

 * The line "I've put a lot of work into it" would later be incorporated into The Eleventh Hour, also written by Moffat, spoken by the true Eleventh Doctor, referring to Earth while responding to the Atraxi's comment that he was not a native of the planet.