[Finwe]

Eregion was a kingdom of sorts, situated almost directly to the West of Moria. It was founded by the Noldor (or the following of Galadriel and Celeborn, depends on where you look) in 750 of the Second Age. For a while, it was under the rule of Galadriel and Celeborn, and everything was peaceful. Other "famous" Noldor who lived there were the Gwaith-i-Mirdain, headed by Celebrimbor, the son of Curufin, and therefore the last Feanorian.

In the Second Age, when Sauron first assumed the fana of Annatar, he first went to Gil-galad and Galadriel, to try to win them over and get in their counsels. Of course, those two were too smart for him, and sent him packing. Then, he got wise, and went to Celebrimbor. This time, however, he didn't offer the almighty prize of world domination/power/etc. etc., he offered supreme knowledge. Annatar claimed that he had been in the train of Aulë in Valinor a long time ago (which was true in a sense, because Sauron had indeed been a Maia of Aulë) and told Celebrimbor that he would give him knowledge that he could only dream of. Naturally, our grandson of Fëanor fell for it.

Annatar also persuaded Celebrimbor to lead a sort of revolt against Galadriel and Celeborn, and take over Ost-in-Edhil, the capital of Eregion. Galadriel fled to Lothlorien along with Celebrian, but Celeborn remained behind (because he would not pass through the Mines of Moria at any cost, since he still hated the Dwarves for sacking Doriath).

With Annatar's tutelage, the Rings of Power were crafted. That story I won't go into because there are many threads devoted to that subject.

After the Elven Rings were forged, and later hidden, Sauron grew very ticked off and decided to attack Eregion with one heck of an army at his back. With so many iron-clad Orc feet marching over it, poor Eriador just didn't have enough Tylenol to get rid of the pain, and it, well, died. So in a nutshell, that is the history of Eregion.