The Miracle

You may be looking for the Miracle which granted immortality to the people of Earth.

The Miracle, as it was named by the inhabitants of Detrios, was a breach in spacetime that lead to a fictional sub-dimension, which appeared over Detrios as a crystallised cloud of fictional energy, giving light to the darkened planet. (PROSE: Head Games)

The Seventh Doctor suggested that this was a semi-natural thing, that could've been caused by the Timewyrm (PROSE: Timewyrm: Revelation) or Mortimus's machinations, (PROSE: No Future) or even as an effect of Gabriel and Tanith's existence, (PROSE: Falls the Shadow) although he knew it was actually caused by Kadiatu Lethbridge-Stewart's time travelling, which had punched several holes in spacetime. (PROSE: Set Piece)

Anyone who visited the Miracle was assaulted by a fictional world based on their own minds, as Bernice Summerfield, Chris Cwej, and the Doctor were when they tried to seal the breach. While Chris ended up back on 30th century Earth, Benny became stuck in Metro City, where she fought the Quiz and met the superhero White Knight, (PROSE: Head Games) whom she met in the Land of Fiction. (PROSE: Conundrum) While the fictional energy from the Miracle could adversely affect the universe, most of it was drawn to Jason because of his connection to the Land, having been the previous Master of the Land. The breach was ultimately sealed, leaving Detrios to fend for itself without light again. (PROSE: Head Games)