Ages ago, the Dark Lord Sauron gave nine kings of men magical rings of immortality.
However, the rings also came with a price.
Their bearers became puppets of Sauron.
They are now known as Ringwraiths.
About the Army
The rules for the Ringwraiths are found in the Mordor supplement.
Although the Ringwraiths are powerful magic users, they grow weaker every turn they are in combat.
They are also not particularly good fighters.
Thus they are best allied with other forces.
I find it useful to mount Ringwraiths on horses.
This gives them a boost in combat, and also effectively increases the range of their spells.
The Witch King is the leader of the RingWraiths.
Khamul the Easterling is not as skilled at magic as the other Ringwraiths.
However, he has the ability to regain will if he wounds opponents.
This makes him very combat oriented.
I advise giving him an armored horse or Fellbeast.
He can also use will to increase his statistics.