Corsairs of Umbar

The Corsairs of Umbar were a group of men descended from the bulk of Gondor's once proud navy, which had aided a usurper in attempting to seize power. Once the rebellion was put down, the bulk of the navy, having sided with him, fled in shame and disgrace. Setting themselves up on the island region of Umbar, they intermingled with Southern peoples living nearby and from this came a new group of people; one that had lost none of their progenitor's nautical abilities, but who were a culture of pirates and slavers. Their ships depended on slave crews to row them, but with a high mortality rate for said slaves, they always needed more slaves if they wanted to keep up their pirating lifestyle, a vicious cycle that terrorized the people of Gondor. Further hurting Gondor was how the Corsairs would take women on their raids to "bolster their numbers".

During the War of the Ring, Sauron recruited them to aid in the sacking of Minas Tirith. Their black ships were to serve as reinforcements in exchange for a nice cut of the spoils. But, warned of the plot by Elrond Evenstar, Aragorn, Legolas, and Gimli intercepted the Corsairs with the Army of the Dead (who had just then joined forces with Aragorn), and presumably killed them all. They then took the ships to the Pelennor Fields and annihilated the terrified armies of Mordor, robbing both them and the Corsairs of their victory.

Trivia

 * In the movies by Peter Jackson, both he and several other members of the production crew made cameos as members of a Corsair ship's crew that was seen briefly in Return of the King. 
 * They are similar to the Ironborn from A Song of Ice and Fire and Game of Thrones, being like them a vicious culture of pirates and raiders who get by on pillaging, raping, sinking ships, and just generally terrorizing other peoples. Both are viewed in-universe as barbarians by most others.