The Time Lords had used the Time Scoop to pull beings out of time and place them in the Death Zone, where they would play "the Game" and try to destroy each other. Daleks and Cybermen were never allowed because they played too well. According to the official history, Rassilon himself put a stop to the Game. From a control room located within the Capitol, only accessible by playing the right tune on the Harp of Rassilon, a person could activate and use the time scoop to capture beings.
When a being had been captured, a figure of them would be created and the figure would be placed somewhere on a model of the Zone, and wherever the figure was placed was where the being would materialise. From the Capitol, the High Council possessed a secret transmat system that was linked to the Death Zone and to the Tomb of Rassilon, which was located in the Zone. However, the transmat system needed activation from the Tomb of Rassilon side, so for a time it remained inaccessible.
Lord President Borusa re-activated the Death Zone. Not knowing Borusa's treachery and only that the Game of Rassilon had begun again, the High Council of Time Lords sent at least one investigator, who died quickly without finding out any information. Borusa made the first, second, third and fifth incarnations of the Doctor play the Game. (When Borusa tried to time scoop the Doctor's fourth incarnation and Romana II, he found himself unable to do so because of a time eddy.)
He also transported Susan Foreman, the Brigadier and Sarah Jane Smith there, while Turlough and Tegan Jovanka remained with the Doctor's fifth incarnation in the TARDIS. Together, as Borusa had planned all along, they braved the Death Zone and managed to find the Tomb of Rassilon safely. Meanwhile, the High Council of Time Lords had recruited the Master to retrieve the Doctor safely out of the Death Zone.
Seven incarnations of the renegade Time Lord Iris Wildthyme, along with their respective companions, once encountered a variety of foes including Ice Warriors, Ogrons, Sea Devils, Mechanoids, Zarbi and Quarks as part of a plan by Morbius to gain Rassilon's gift of immortality. (PROSE: The Scarlet Empress)
When Rassilon was resurrected to lead the Time Lords in the Last Great Time War, he turned Borusa into a possibility engine that could see different times and possibilities and often asked his advice. He kept Borusa in his tomb due to how hideous the Time Lord High Council found this. His earlier experiments were left in the Death Zone.
The War Doctor and Cinder entered the Death Zone so the Doctor could see Borusa, but he was unable to transport himself closer. They encountered a lizard-like creature from Gallifrey's past and a large monster which chased them but they escaped into a cliff where they encountered some of the Time Lords Rassilon had experimented on. (PROSE: Engines of War)
Physical environment Edit
The Death Zone had mountains, marshes, and at least one paved road. The fog was often so thick that one couldn't see where one was going, and red thunderbolts which would kill instantly frequently fell from the sky. (TV: The Five Doctors) It was about the size of North Wales, despite those on Gallifrey believing it to cover a great distance. Both Iris Wildthyme and the Doctor knew this. (PROSE: The Scarlet Empress) The Second Doctor described it as "the perfect battle ground". (TV: The Five Doctors) It was inhabitated by animals, often dangerous, from different time zones. (PROSE: Engines of War)
Alternative "pieces of the board" to the Death Zone existed and could be exchanged with each other. The sections that were not in use were held in a pocket dimension from which the players could not escape. (AUDIO: The Five Companions) The entire Zone existed within a time bubble. (PROSE: Engines of War)