Eregion or Hollin was a kingdom of the Noldorin Elves in Eriador during the Second Age, located near the West Gate of Khazad-dûm under the shadow of the Hithaeglir (Misty Mountains). Its capital was Ost-in-Edhil.
Eregion was populated by Ñoldor, and for a while ruled by Galadriel and Celeborn, until they left for Lothlórien on the other side of the Misty Mountains. The Elves of Eregion lived in harmony with a Dwarven civilization, trading freely with the Dwarven kingdom of Khazad-dûm, or Hadhodrond in the Elven tongue.
After Galadriel and Celeborn had left, Eregion was ruled by Celebrimbor, a descendant of Fëanor. Under his rule the Elves of Eregion became friends with Annatar, Lord of Gifts, and created the Rings of Power. When Annatar was revealed as the Dark Lord Sauron the Elves of Eregion tried to keep the Rings from falling in Sauron's hands, but only managed to rescue Vilya, Narya, and Nenya (the Three Rings of the Elves). Eregion was utterly destroyed, and its survivors fled to Lindon, Lothlórien, and the refuge of Imladris (Rivendell).
In the Third Age, Eregion was a pleasant but unpopulated land.
Eregion should not be pronounced as in English region: the g is the g in get, so a closer pronunciation is eh-REH-gee-on (io is not a diphthong in Elvish).