The Iron Crown was forged by Morgoth in Angband after his return to Middle-earth at the beginning of the First Age to hold the three Silmarils and to symbolize him as King of the World. The Silmarils were set in the Crown, fastened by iron claws.
In the quest for the Silmaril, as told in the tale "Of Beren and Luthien", Beren pried one Silmaril loose from the crown. After Morgoth's defeat in the War of Wrath, the other two jewels were removed and the crown was beaten into a collar for his neck.
In the Third Age, during the Witch-King's campaign against Arnor, he wore an Iron Crown, and his supporters often called him "The Iron Crown" or "The King of the Iron Crown".