The Arkheon Threshold was a small tear in time that led to the world known as the Planet of Ghosts. The time rift created spectral visions. It was a rumour of a future war between humanity and the Daleks. People believed it had been destroyed forty years ago.
The planet was destroyed, but the Threshold remained. It was an objective of the Dalek Empire. They sought it on the planet Hurala. The tide of the war was turning against them. They sought to win by altering time itself.
After journeying to the planet Arkheon, the Doctor and his companions were captured by the Daleks, who had laid a trap for them. The Dalek Inquisitor General was responsible for a massive mining operation on Arkheon using captive colonists from Auros. The Inquisitor General hoped for his race to become masters of time by using the Threshold to enter the Time Vortex. They would use a Large Chronon Collider, five storeys high, to break upon the chronic schism. They were uncertain it would work, so Dalek X tried to make the Doctor use his TARDIS for this. The Daleks were defeated and the Threshold was permanently sealed by the Doctor. (PROSE: Prisoner of the Daleks)