At the far eastern end of the heights of Dorthonion, the mountains of that land sloped swiftly downward, and fell into a string of hills running onward to the east. Where the mountains of Dorthonion came to an end and the line of eastern hills began there ran a narrow gorge, known as the Pass of Aglon. This gorge had high, steep walls, especially on its cliff-like western side. It provided a way through the highlands from the plain of Lothlann in the north through to the land of Himlad in the south.

After the Noldor returned to Middle-earth, the Pass of Aglon fell within the lands claimed by the Sons of Fëanor. It lay under the shadow of Maedhros' fortified Hill of Himring, though the gorge itself was under the control of Maedhros' younger brothers Celegorm and Curufin. As a possible way through the guarded March of Maedhros, the pass was heavily fortified and strongly guarded against any possible incursion by Morgoth's forces from his northern domains. The guards of Aglon also kept watch on the lands of Himlad southward of the pass, and the fords of the Arossiach that ran into those lands from the west.

The Pass of Aglon was held by the Sons of Fëanor throughout the long Siege of Angband, but in I 455 Morgoth launched the Dagor Bragollach, the Battle of Sudden Flame, against the Elves. Hordes of Orcs came against the defenders of the pass and forced their way through, and the brothers Celegorm and Curufin fled into the south. Maedhros on the Hill of Himring, however, was able to defend his position, and he was reinforced by survivors escaping the fighting elsewhere. Together the Elves and Edain were able to recapture the Pass of Aglon, and hold it against the hosts of Morgoth. When the battle finally ebbed, Maedhros and his brothers refortified the pass, filling it with hidden traps for unwary invaders.

The final fate of the pass is not expressly described, but the fact that the Sons of Fëanor were said to hold it only 'for a while'² implies that the Orcs eventually managed to force a way through. The pass is not mentioned in records after this time, so it is hard to be sure whether the Elves held the pass for a matter of hours, or for years. At the very least it seems likely that the Orcs broke through after Morgoth's victory in the Nirnaeth Arnoediad, seventeen years after the Dagor Bragollach.