The Nightmare

The Nightmare was the distillation of all that was evil in the Detective, and a composite of his every dark thought, created by the Time Lords using black operation technology. (COMIC: The Detective)

He later became known as the CyberKing, after creating the Cybermen from this universe of Earth's twin planet Mondas. (COMIC: The Detective, WC: Detective Chronicles) He was captured and converted into a Cyberman many centuries later, who then led the CyberSupreme in war against the CyberRegime. (TV: Death in Heaven, WC: Detective Chronicles)

After the Cyber-Wars, he lost his cyberman's body and was download into a machine, becoming the Cyber-Planner, who tried to overtake the Eleventh Doctor's mind in an attempt to resurrect the Cybermen. (TV: Nightmare in Silver, WC: Detective Chronicles)

He was eventually defeated when the Doctor distracted him by claiming to have a way to avoid defeat in chess, and then removing him from his brain with a modified hand pulse. He was then redistributed within the Cyberiad and was presumably destroyed when Hedgewick's World was imploded by command of Emperor Ludens Nimrod Kendrick Cord Longstaff XLI. (TV: Nightmare in Silver, WC: Detective Chronicles)

Personality
The Nightmare was similar to several of the Detective's incarnations, cunning, ruthless and arrogant. (COMIC: The Detective) His desire to protect people went beyond individuals. He viewed himself as a great man who is on a search for truth, and who wants nothing more than to help people and humanity. (COMIC: The Detective)

He was blind to the idea that his help is purely conditional: if the person he's helping doesn't completely depend on him or reveal the entire truth about their background to him, he will turn on them completely and make them his enemy. He often blamed others for not coming to him right away with the truth, or for not asking for his protection. (COMIC: The Detective)

He also believed that the ends justified the means, willing to kill others for mistakes they had made or dangerous experiments that they had been involved in even if they regretted their actions and showed no sign that they would repeat their errors, as he was unforgiving and merciless. (COMIC: The Detective)

Cyber-Planner / "Mr Clever"
Unlike most Cybermen, he displayed a distinct personality and emotional responses, likely due to being influenced by the Doctor's formidable persona. This could be seen manifesting in the occasional use of such phrases as "Allons-y" or "Fantastic!", such being often used by the Doctor's previous regenerations, as well as his decision to name itself and the choice of name. (TV: Nightmare in Silver)

He also shared the slightly manic attitude of the Doctor; leaping on tables and twirling on the spot when excited. (TV: Nightmare in Silver)

He was shown to have his own distinct personality. He was extremely excited to discover the Doctor was a Time Lord. He was quite arrogant, not even considering the potential of losing, absolutely resolute that there was no way he could lose. He was ruthless and lacked any sense of honour, intending to go back on his agreement with the Doctor and kill everyone anyway. (TV: Nightmare in Silver)

CyberKing
As the CyberKing, he had increased strength, speed, teleportation, time travel, reactive adaption and telepathy. He could also cyber-conversed individuals via touch and mentally control cybermen. (TV:  Death in Heaven)

"Mr Clever"
After overtaking the Doctor's body, he was capable of controlling parts of the body, such as the Doctor's left arm. Although, he was able to takeover nearly half of the Doctor's mind, the Doctor retained the other half of it; with a small percentage of his mind unclaimed by either. The two then decided to play a game of chess, while also attempting to lead their respective allies to victory in the physical world. (TV: Nightmare in Silver)