Celestial Toymaker

The Celestial Toymaker, also known as the Crystal Guardian, the Mandarin, or simply the Toymaker, was a powerful being who ensnared sentient beings in apparently childish games, with their freedom as the stakes. However, the Toymaker hated to lose and the games were always rigged in his favour.

Characteristics
According to the Sixth Doctor, he originated in another universe before this one and was hauled into the Doctor's universe by some kind of catastrophe. Because of this, the usual laws of physics didn't apply to him. The Toymaker himself stated that, while he had used his powers for other things in the beginnings of the universe, he eventually got bored of thousands of millennia of pointless creation and pointless destruction, and found a new source of amusement — games. (TV: The Nightmare Fair)

Within his realm, the Celestial Toyroom, the Toymaker commanded immense powers, but they were limited by the rules he set for any particular game, although he could bend these rules or "forget" to mention them to his opponents if he so chose. He himself was immortal and invulnerable, and appeared capable of space and time travel at will. For some reason he chose to appear in the guise of a middle-aged Caucasian human dressed as a Chinese mandarin.

During the course of a game, one of the players might die outright or they might lose, in which case, the Toymaker would have total control over their life and personality, perpetually. (TV: The Celestial Toymaker)

History
During the latter days of his first incarnation, the Toymaker drew the Doctor's TARDIS back to his realm and made the Doctor and his companions play his games again. This time the Toymaker arranged things so that even if they won, the Toyroom would vanish completely at their moment of victory, leaving him the only survivor and the Doctor and his companions his subjects forever. The Doctor outwitted the Toymaker again and escaped, leaving his realm in chaos. At the end of this adventure, the Doctor believed, incorrectly, that since he had won, the Toyroom no longer existed. (TV: The Celestial Toymaker)

The Toymaker, now calling himself the Mandarin, appeared in Blackpool using a thrill ride, Space Mountain, as his base of operations, and manufactured deadly video arcade games. He was stopped by the Sixth Doctor and placed in an impenetrable force field. Because the Mandarin's mental energy maintained the force field, the Doctor had set it up so that he could never escape it. (TV: The Nightmare Fair)

Behind the Scenes

 * According to Donald Tosh, the commissioning script editor and (uncredited) co-author of The Celestial Toymaker, the intention was that the Toymaker was, like the Monk who had predated him, a member of the Doctor's own race. (BBC DVD: The Time Meddler) This fact has not been followed upon in any narratives featuring the Toymaker to this day, as that race was later developed as the Time Lords whose abilities differed substantially from the Toymaker's.
 * In an interview, Peter Cushing stated that he believed that his human incarnation of the Doctor was canon, and that his film version of the Doctor and the television Doctor were bridged together by the Celestial Toymaker. His theory was that his Doctor is a future incarnation kidnapped by the Toymaker, who "wiped his memory and made him relive some of his earlier adventures."

Season 3

 * The Celestial Toymaker

Season 23

 * The Nightmare Fair