The chain which Aulë wrought to bind Melkor.

He was bound with it twice, once after the Battle of the Powers (before the First Age) after which he was chained in the Halls of Mandos for three ages. Then again, after the War of Wrath (583 of the First Age), he was cast into the Void beyond the world, bound in that same chain.
Encyclopedia entry originally written by Hathaldir