The Siege Perilous is an artifact in the Marvel Universe that acts as a portal to an unknown dimension where judgment would be passed on those that entered. It was a small pocket-sized artifact, but when it was powered up, it became a large portal. The X-men entered it to stop a dangerous foe in Dallas, Texas, thinking it would lead to the death of their advisory, but also their own deaths. They where however brought back by Roma after the X-men relocated to Australia and let the world believe they actually died.
Roma gave the portal to the X-men, whom took possesion of it since then. The X-men used it a second time against the Master Mold whom was battling the X-Men when he absorbed the nearby Nimrod, a Sentinel from the future. Master Mold was forced into the Siege Perilous. What emerged was the biological sentinel Bastion, who had no memory of his past. Unfortunately, the X-man Rogue was thrown into the portal at the same time and she seemed to have vanished from the earth.
Eventually, most of the X-men entered the Siege Perilous when they tried to flee from the Reavers in a battle they knew they could not win. The team was scattered across the planet, and most of them lost their memories for a time. It caused the disbanding of the X-men team that was located in Australia at the time. Just after the X-men had fled, Donald Pierce of the Reavers picked up the now pocket-sized artifact, and destroyed it.
The Siege Perilous first Appeared in Uncanny X-Men #227.
King Britain, Queen Medusa, The Mar-Vell Child, Captain America and The Black Knight ( Ahura Boltagon) of Earth-9997 once used the Siege Perilous gateway in Darkmoor to visit Merlin, Roma and Earth-9997's Psylocke and found themselves within The Omniverse itself. Suggesting that The Siege Perilous can be used as a doorway into every alternate reality, parallel universe and possible Pasts, Presents and Futures in existence. Some of the accessible realities seen by the group while they were within The Omniverse were the MC2 reality, Killraven's Future, Mutant X and Days of Future Past.
Ultimate Spider-Man: Web Warriors
In the four part Spider-Verse episode, the Goblin used the Siege Perilous to visit alternate Earths to gather DNA from alternate Spider-Men. It appeared as a smartphone sized artifact that opened purplish wormholes to the other Earths. It required a lot of energy to operate, supplied by Electro in this instance. It was shown to be able to provide access to versions of the 2099, Noir, and Ultimate realities, among others.