Sangahyando was a great-grandson of Castamir, who had usurped the throne of Gondor. After the rightful King, Eldacar, had regained his realm, Castamir's sons fled to the Haven of Umbar, where they formed a refuge and base for the enemies of Gondor. Sangahyando was one of their descendants, and with another, Angamaitë, he led a raid on Pelargir nearly two hundred years after their grandfathers had been driven from that city into exile. Their raid was a stunning success, and they succeeded in slaying Gondor's King at that time, Minardil.
Sangahyando's name is not easy to translate into English. The conventional translation is "Throng-cleaver", but "throng" is only an approximate translation of Quenya sanga. In this context, a sanga was a body of soldiers in close formation, so Sangahyando's name refers to him cutting into his enemies' defences.