Angainor

After the Elves awoke at Cuiviénen, the Valar went to war against Melkor. In the Battle of the Powers that followed, Melkor's great fortress of Utumno was destroyed, and Melkor himself was captured and bound with a chain created by Aulë. That chain was known as Angainor, and once chained with it, Melkor was carried back to Valinor and shut in the Halls of Mandos for three ages.

After the three ages had passed, Melkor was released, but he soon turned back to evil. Destroying the Two Trees and stealing the Silmarils, he established himself in his old stronghold at Angband, and began an age-long reign in Middle-earth. His time as Dark Lord was ended when the Valar again came against him in the War of Wrath, in which Angainor was used to hold Melkor captive once more.

The Silmarillion tells us little about Angainor beyond its bare existence, but the idea of the great chain that bound Melkor goes back to the earliest of Tolkien's works. In The Book of Lost Tales 1, we're told that Angainor was made of unique unbreakable alloy known as tilkal, created by Aulë, that appeared green or red according to the light that shone on it. The name Angainor, at least according to that early version of the story, means 'the Oppressor'.