Barad-dûr: built by Sauron with the power of the One Ring, during the Second Age. The building took six hundred years to complete; it was the greatest fortress ever built since the Fall of Angband, and much of Sauron's personal power went into it.
The Nazgul. Called the Ulair in the tongue of the elves, these nine once bore Rings of Power. But they could not see the Enemy for what He was. Now they are His fingers, leeching foul darkness into Arda.