Second Age Edit
Eregion was, from SA 750, ruled by Galadriel and Celeborn from its capital of Ost-in-Edhil. After SA 1350, they crossed the Misty Mountains to Lothlórien, their new kingdom.
From SA 1350, the Gwaith-i-Mírdain under Celebrimbor, the last of the House of Fëanor left in Middle-earth, ruled Eregion. Annatar, the Giver of Gifts, befriended Celebrimbor and aiding him in the making 19 of the Rings of Power. Celebrimbor made the three rings of the Elves in secret and these were never touched by Annatar's hands.
When Annatar was later revealed to be the Dark Lord Sauron, Celebrimbor and his people tried to stop the Rings from entering Sauron's possession but could only rescue the Three: Vilya, Narya, and Nenya. Eregion itself was attacked and destroyed in the War of the Elves and Sauron in the year SA 1697. The few survivors of the war found refuge in either Lindon or Lothlórien, while others led by Elrond founded Rivendell.
Third Age Edit
In the Third Age, by the time that the Fellowship of the Ring reached Khazad-dûm, the land was pleasant but unpopulated. Only the scattered ruins of Elven cities and the few remaining holly trees indicated its former glory.