Castamir is often referred to as 'The Usurper'.

He was a descendant of King Calmacil and when Eldacar ascended to the throne in Gondor, there was a rebellion against him, claiming that Eldacar had no right to the kingship because he was of only half-Gondorian blood.

Castamir sought to claim the throne himself and he led a great uprising against Eldacar, finally sending the rightful king into exile, taking the kingship for himself.

Castamir ruled for 10 years, until Eldacar was able to rebuild his strength and return to reclaim the throne.