Canyon of Yon

At the base of the Mountain of the Sky sits a canyon. The depth of the canyon can be calculated, but not often reached. Many who have traveled into the canyon have never been seen from again. At least, not in the Material Plane. The Canyon of Yon acts as a breaking point between the realms, a point where the walls of the different planes of existence are close and weaken. The plane that seems closest to the Canyon of Yon is that of the Nine Hells, with many a doorway having been opened from there to the Material plane through the Canyon. 
The Canyon of Yon has been seen before as a point of great importance in wars. During the Shadow Wars, the necromancer Sah'roth attempted to use the Canyon to spread his army of the Forgotten (dark spirits long lost to the world reconstituted in desecrated forms) throughout the planes. He faced opposition though, as a union of Humanity, Elvenkind, Dwarvenkind, Celestials, and Infernals took arms against the mad necromancer and his forces of darkness. Through trial and tribulation, they were able to banish the Forgotten back to where they came, but great loses were made. The Human representative, Garrus Bridan, and the dwarven representative, Dimano Floornt, were captured and killed by Sah'roth himself. 
The Shadow Wars ended when Sah'roth was felled by the Elven representative, Lady Inara Reiner. She broke through the lines of battle as Sah'roth's army pushed upon the Canyon. She carved her way through to Sah'roth, and emerged with his head. The loyalists of his army were swiftly eradicated, and peace was returned to the lands. As a last act of the union of the cardinal races, the gates to the Canyon of Yon were sealed. The Church of Galaran, God of Justice and Order, and the Army of Axiom, the holy city that sits near the peaks of the the Mountain of the Sky, continue to guard the Canyon of Yon to this day.