The son of KingValacar of Gondor and Vidumavi. Vidumavi was the daughter of Vidugavia of the Northmen, the self-styled King of Rhovanion, and so Eldacar was of only half-Gondorian descent, a fact that led many to doubt Eldacar's right to rule. After his father's death, he sat on the throne for just five years when civil war broke out in Gondor. Ultimately, he was deposed by his Captain of Ships, Castamir, who took the throne himself.
Eldacar escaped into the north, where he spent ten years raising an army before he returned to reclaim his throne. In a great and bloody battle at the Crossings of Erui, Castamir was defeated and the remnant of his forces fled into the south, to Umbar. Eldacar ruled for another forty-three years after this war, known as the Kin-strife. His eldest son Ornendil had died at Castamir's hands, and he was succeeded by his second son, Aldamir.
The date of Eldacar's birth appears only in The History of Middle-earth volume 12, The Peoples of Middle-earth. It cannot therefore be considered completely reliable.