Himring was a hill in the northeast of Beleriand. It was east of the Pass of Aglon and west of Maglor's Gap. The greatest hill of that region, it was wide-shouldered, treeless, with a flattened summit, and surrounded by numerous lesser hills. After the drowning of Beleriand at the end of the First Age, the peak of Himring remained above the waves.
The fortress of Maedhros
The fortress on Himring was built when the Sons of Fëanor went east shortly after Maedhros, eldest of the Sons of Fëanor waived his claim as High King of the Noldor in favor of Fingolfin. It was the chief fortress of Maedhros, from which he guarded the north-eastern border region that became known as the March of Maedhros.
The fortress stood firm through the Dagor Bragollach, and many survivors from the surrounding regions, including Maedhros' brother Maglor, rallied there. In the battle the Orcs had taken the Pass of Aglon, but Maedhros was later able to retake the Pass using the forces from Himring.
Himring's fortress remained for centuries, until the Nirnaeth Arnoediad; where the sons of Fëanor forsook it. After the Nirnaeth, the hill of Himring became garrisoned by soldiers of Angband.
The Island
After the War of Wrath, when the western lands were flooded, the plains about the hill were drowned and the top of the hill was all that remained of Himring. Left standing as an island, Himring lay off the north-west coast, about twenty-five miles out from the shores of northern Lindon.