The Gwaith-i-Mírdain was a brotherhood formed by Celebrimbor and the other master craftsmen of Eregion in the Second Age and their capital was Ost-in-Ethil. They stood close to the Dwarves of Khazad-dûm and their mithril mines. They cooperated for about a thousand years, and their rare friendship was the closest between Elves and Dwarves in history. Celebrimbor helped to construct the famous and magical West-gate of Moria. They were said to be the most talented artisans to have worked since the time of Fëanor himself (who was Celebrimbor's grandfather).
Their skill was subverted by Sauron, in the guise of Annatar the "Lord of Gifts" in S.A. 1200, who focused their abilities on forging the Rings of Power in S.A. 1500. Celebrimbor presented King Durin III with a Ring of Power. The Rings were completed in S.A. 1590, when Sauron betrayed them and forged the One Ring. However the Gwaith realized his plan and denied to hand him their Rings, resulting in the War of the Elves and Sauron. Eregion was devastated and the survivors of the Gwaith retreated to Rivendell