The Nazgûl (or named Ringwraiths, sometimes written Ring-wraiths; also referred to as the Black Riders or as the Nine; or the Nunbolg; or the Ulairi in Quenya) were the dreaded ring-servants of the Dark Lord Sauron in Middle-earth throughout the Second and Third Ages, and in the later years of the Third Age they dwelt in Minas Morgul and Dol Guldur.
These are one of the major powers of Mordor, hero units whose power is tied to Sauron, and grow in strength as he grows in strength. To suit this end, the nazgul can increase Sauron's power twice (three times, in the upcoming edition). By summoning your first ringwraith, Sauron gains one level, and by having all nine on the field, Sauron gains one more level. The nine are divided into two groups. A hero-battalion of five can be summoned from the citadel. The remaining four ringwraiths are controlled individually; two of them are recruited from the citadel, one from the Dol Guldur outpost fortress, and one from the Minas Morgul outpost fortress.
As Mordor is about swarming enemies with masses of foot-soldiers that accompany huge beasts and terrifying monsters, the ringwraiths are one of the ideal super units that are made to fight among masses of orcs. For one, they are among the few cavalry units that Mordor can access, and they are quite strong in that respect, especially when all together in a single group. Their very presence weakens enemy units, and they can disrupt enemy formations with their horrible shrieks. The nazgul can also use their morgul blades to deal immense damage to single targets such as hero units and monsters, and when enemy units die to their morgul blades, their spirits can serve as great scout units, or they can be transformed into powerful warrior-ghosts, made in the image of the ringwraiths themselves.
With "His Deadliest Servants" active, the unique ringwraiths all have access to two new forms depending on what is needed at the time. As ring-hunters, the nazgul have access to new powers that help them remain alive, and they can also detect invisible units, giving them the role of tracking down the One Ring, and slaying its bearer. Alternatively, the four unique nazgul can also be given special attire that boosts their attributes so that they are more suitable for combat, and giving them even more unique powers and roles.