Gates of Elysium

Created when something unknown to even the Time Lords gouged reality in a place in space, the Gates of Elysium were an antimatter cascade, a pit of clashing realities. Unfolding as if it was a waterfall of unspace where photons struggled like flies trapped in fossilised resin, its colours were shifting and bright enough to cause a migraine. The General found themself unable to find the proper words to describe the cascade. Other than simply saying it looked like what insanity would look like, the General compared it to a "reflection of mythical hell", as if the the universe's forces of heat, gravity, and physics had betrayed each other. Ultimately, even though Gallifreyan language was made to be used on a celestial scale, not even its words could summarize the Gates of Elysium. (PROSE: The Third Wise Man)

History
According to the General, if the gates had been purposely made, they would have had to have been made by something beyond even the Time Lords, something that looked down on them like they looked down on all other civilisations. Nevertheless, it was also possible the gates had been somehow accidentally made. The unspace spilling from the gates killed atoms themselves for one hundred light-years in each direction. (PROSE: The Third Wise Man)

During the first year of the Last Great Time War, (TV: The Stolen Earth) the gates were the location where Davros tried to kill the Nightmare Child, which the Time Lords confused for a space station impossibly hanging above the gates at first. When the child unleashed itself, it spread like a swarm, consuming Daleks and TARDISes alike above the gates. Davros had also sent a distress signal to the War Doctor, who attempted to save him. Davros only wanted the Doctor to watch as the Nightmare Child consumed his command ship. The gates closed behind them and vanished. (PROSE: The Third Wise Man)

The Doctor believed that Davros had been killed, however Dalek Caan had actually saved him, at the cost of his own sanity. (TV: The Stolen Earth) Meanwhile, even though the Nightmare Child had been thrown through the gates, its antimatter baptism (PROSE: The Third Wise Man) did not kill it, for it returned to become a threat until the final days of the War. (TV: The End of Time)