In the time of the Kin-strife, the civil war of Gondor, KingEldacar was deposed by Castamir and his eldest son Ornendil was slain. Eldacar later recovered his rightful Kingship, but because of Ornendil's early death he was succeeded by his younger son Aldamir. Aldamir's reign was marked by war with the southern lands, including the Haven of Umbar that had been seized by Castamir's supporters. Aldamir was slain in that war and succeeded by his son Vinyarion, who avenged his father's death by conquering the Haradrim, after which he ruled as Hyarmendacil II.
Notes
1
The date of Aldamir's birth appears only in The History of Middle-earth volume XII, The Peoples of Middle-earth. It cannot therefore be considered completely reliable.
2
Aldamir's father Eldacar gave both of his sons 'tree' names (Aldamir's elder brother Ornendil had a name meaning approximately 'tree friend'). Eldacar's mother Vidumavi had a name meaning 'wood maiden', and her father was Vidugavia or 'wood dweller', so Eldacar's choice of 'tree' names for his sons perhaps followed this tradition of his mother's people. There may equally be a connection to the White Tree, which still grew in the courts of Minas Anor at this time. The element -mir, meaning 'jewel' or 'precious thing', was a common name ending among the Gondorians.
3
The fact that Aldamir had an elder sister can be inferred from a comment in The Heirs of Elendil in volume XII of The History of Middle-earth, where Aldamir is described as the 'second son and third child' of Eldacar.