Sangahyando was a great-grandson of Castamir the Usurper who had taken the throne of Gondor from its rightful king.

When Castamir was defeated, his sons fled south to Umbar where they gathered to them other enemies of Gondor. They dwelt there for many years, and though they were driven from Pelargir, still Umbar remained their stronghold.

Approximately 200 years after the defeat at Pelargir of their grandfathers, Sangahyando and another man named Angamaitë regained the city and slew the King of Gondor at that time, Minardil.
Encyclopedia entry originally written by sairina_star