Corsairs of Umbar - After the civil war, the followers of Castamir fled south to Umbar in Gondor's ships.

 

"...but Castamir's sons escaped, and with others of their kin and many people of the fleets they held out long at Pelargir.

 

"When they had gathered there all the forces that they could (for Eldacar had no ships to beset them by sea) they sailed away, and established themselves at Umbar.  There they made a refuge for all the enemies of the king, and a lordship independent of his crown.  Umbar remained at war with Gondor for many lives of men, a threat to its coastlands and to all traffic on the sea.  It was never again completely subdued until the days of Elessar; and the region of South Gondor became a debatable land between the Corsairs and the King." ~The Return of the King (Appendix A)