His reign saw the already lingering unrest explode, for his mother, Princess Vidumavi, was a daughter of the Northmen King Vidugavia, a lesser Man. After taking the throne in 1432 T.A., he reigned for five years, until he was deposed and driven from Osgiliath into exile to Rhovanion by his distant relative, Castamir the Usurper, in a civil war called the Kin-strife.
In exile, he slowly gathered support from the Northmen and the Dúnedain from the northern regions of Gondor (who hated Castamir, who executed Ornendil, Eldacar's firstborn son). In 1447 T.A., he marched south with his army and slew Castamir personally in battle at the Crossings of Erui. But Castamir's sons escaped and Gondor's purest blood was spilt. He besieged Pelargir where Castamir's sons held out. Eldacar had no ships at his disposal (for Castamir had been the Captain of Ships of Gondor) but after a years siege Castamir's sons finally fled to Umbar.
In the event Eldacar reigned until 1490 T.A. dying at the respectable age of 235, thus negating the charges of the Rebels that the Royal blood had been adversely affected by mingling with 'lesser men'. He was succeeded by his second son, and third child, Aldamir.