Corsairs of Umbar
The Corsairs of Umbar were sea-raiders and pirates of the Haven of Umbar. Umbar was an old Númenórean haven settled by the "King's Men" or Black Númenóreans, a proud faction loyal to the King and opposed to the divine authority of the Valar.
After the Downfall of Númenor, the Umbar settlers further descended into evil, and were called the Black Númenoreans. They took to pillaging and piracy along the coasts of Gondor. During the Kin-strife, the defeated rebels of Gondor fled to Umbar — by this time Umbar became the hated enemy of Gondor.
By the time of the War of the Ring, the Corsairs had mixed with the Haradrim, becoming a mixed people where Númenórean blood was mostly gone. During the Battle of the Pelennor Fields, a fleet of Corsairs was raiding Lebennin when Aragorn captured their ships and rowed them to Minas Tirith to relieve the siege of the city.
In The Lord of the Rings: The Battle for Middle-earth II, corsairs are from the Mordor faction, and are equipped with knives and fire-bombs.