Annúminas was a great city of the kings of Arnor, on the shores of Evendim. It was deserted however, during the war against the Witch-King in the early Third Age. One of the Witch-King's Lieutenants was an elf, corrupted by one of the minor Rings of Power, Narchuil. Her name is Amarthiel, the Champion of Angmar. In an attempt to gain his daughter back, the father of Amarthiel destroys Narchuil in two pieces and hides them, after the fall of the Witch-King during the battle of Fornost. He fails however, and Amarthiel disappears.
During the War of the Ring, Sauron sends one of his Lieutenants to rebuild the kingdom of Angmar. The steward of the Witch-King is the wraith Mordirith. Books 1 to 8 of the epic storyline of the game is about the player against the armies of Angmar, and at the last chapter you manage to strike a severe blow against Mordirith. Mordirith pulls back to Carn Dûm. With Mordirith in a weakened position, Amarthiel shows up again, quickly grabbing the palantír of Carn Dûm which previously belonged to Mordirith. With the palantír she seeks Narchuil, her lost Ring.
Being a rival of Mordirith, Amarthiel chooses to reside in Annúminas. During the storyline in Annúminas, with the help of the Dúnedain you eventually capture Mordrambor, one of the Black Númenorian lieutenants of Amarthiel. After a vision in the palantír, Amarthiel sends Mordrambor to the Trollshaws to find Narchuil. You grab and destroy the palantír of Annúminas, however.
Mordrambor finds one of the two pieces of Narchuil and returns it to one of the fortresses in Angmar, for study. After a succesful raid you capture the piece. Laerdan, the father of Amarthiel, tells you where he hid the other piece - he gave it to the Last King of Arnor. That means the piece is in the ship that sunk in the barren ice-lands of Forochel. I don't know how it continues (my game card was up ), but eventually you gain the other piece, and as the rivalry between Mordirith and Amarthiel grows into civil war, attempt to reforge it in the Eregion Forges.
Although they fight both for the Witch-King, Amarthiel and Mordirith could never ever be on the same team, because they are eachothers rivals and are currenty (in the MMORPG Lord of the Rings Online) in Civil War. Instead, you can only build one of the two - Amarthiel or Mordirith.
Pictures of the two:
Amarthiel, Champion of Angmar
The Wraith Mordirith, Steward of the Witch-King (a.k.a. The False King)