The Lost Men of Udûn, not to be confused with Ringwraiths, were the greatest strategists in all of Middle-Earth. They were once military heroes of Gondor, but were corrupted by Sauron and joined his armies. They were excellent at countering the military tactics of Gondor. They forced Gondor to abandon its normal strategies and make major decisions on the spot. They primarily use the weapons of Gondor which they were familiar with.
As said above, the Lost Men of Udûn were once great military leaders in Gondor's armies. Later in the War of the Ring, after his defeat at Helm's Deep, Sauron decided that he needed generals who could think like the enemy. Despite how the Uruk-hai were such great warriors, they still needed guidance. He ordered his spies in Gondor to find who the greatest leaders in Gondor. After he had the information, Sauron captured the men and corrupted them in Udûn. His plan proved a success. The Lost Men of Udûn, as they were now known, were able to finally capture Osgiliath. The Men also recruited the Corsairs of Umbar for the assault on Minas Tirith. The battle would have been won for Sauron if Aragorn had not unexpectedly arrived with the Armies of the Dead. Most of the Lost Men were killed in the battle, and the others were captured by Gondor.
Many of the members of the Lost Men of Udûn claimed to have forgotten their own names when questioned. However, it is possible that the Mouth of Sauron was a Lost Man of Udûn. The only known member is Anardil the Destroyer.