Maglor's Gap was the open area between the hills surrounding Himring and an extension of the Ered Luin just north of Mount Rerir. It was the widest break in the northern mountain-fences of Beleriand.
Due to the open terrain it was through this gap that Orcs entered East Beleriand before the Dagor Aglareb. In response the Noldor kept cavalry in the nearby plains, Caranthir fortified the mountains to the east of the gap (establishing his base beside Lake Helevorn), and Maedhros constructed his stronghold on the Hill of Himring to the west.
Most of these preparations came to naught during the Dagor Bragollach. The Orcs overwhelmed the Elves' cavalry upon the Lothlann and the dragon Glaurung passed through Maglor's Gap to destroy all of the land between the arms of Gelion. The fortresses upon Rerir's western slopes were destroyed and Thargelion was ravaged. Only Himring was successfully defended, to which Maglor himself fled along with most survivors of the battle in that region.
- J.R.R. Tolkien, Christopher Tolkien (ed.), The Silmarillion, "Map of Beleriand and the Lands to the North"
- J.R.R. Tolkien, Christopher Tolkien (ed.), The Silmarillion, "Quenta Silmarillion: Of Beleriand and its Realms"
- J.R.R. Tolkien, Christopher Tolkien (ed.), The Silmarillion, "Quenta Silmarillion: Of the Ruin of Beleriand and the Fall of Fingolfin"