The Force

 was one of the fictional creations of Jason after he was made Master of the Land of Fiction. It was a sentient energy being.

Landing in Arandale in 1959 during a meteor shower, it became trapped beneath Arandale Keep. It possessed the unborn Phil Chambers, influencing him over the years. As an adult, Chambers killed and drained the blood of a number of people to make his body a suitable host for the Force. When it's goal was finally achieved, it freed itself and fully inhabited Chambers.

Knowing where he was shortly after arriving, the Seventh Doctor tricked Jason into making the Force be composed of McAllerson's Radiation. When the Force, through Chambers, tried to kill the Doctor and his friends with a burst of that radiation, the truth was revealed. Instead of being lethal to people as the Doctor had led Jason to believe, it disrupted the control systems of the Land, allowing the Doctor and his companions to escape. (PROSE: Conundrum)