Castamir “The Usurper” (TA 1259-TA 1447) was the 22nd King of Gondor.
As the grandson of Calimehtar he had a legitimate claim to the throne, so rose up and usurped Eldacar from the throne of Gondor.
The rebellion began to brew as Eldacar was not descended from pure Numenorean. This was unheard of for the heir to the throne, as many of the older dignitaries of Gondor were worried that the “mixing” of races would lead to reduced lifespans.
As the Civil War raged on Castamir ousted Eldacar to flee to his kinfolk the Northmen. Castamir was seen as ruthless, causing huge damage to Osgiliath along with executing Eldacar’s son Ornendil.
After Eldacar’s return with the Northmen, Castamir was slain during the Battle of the Crossings of Erui, Castamir’s sons withdrew their forces eventually travelling to Umbar. Umbar then became a haven for those who were enemies to Gondor.
Trivia– Castamir is not a glossed name, however in Quenya it is thought to mean something along the lines of “Jewelled Helmet” or just “Jewel”.