Rómendacil had intended that Valacar should learn something of the language and culture of the Men of Rhovanion, but he had not foreseen how strongly his son would be drawn to the Northmen. Valacar went so far as to wed Vidumavi, daughter of Vidugavia, a union that would bring great troubles in the future.
The Northmen did not share the long lives of the Gondorians, and Vidumavi died even before Valacar had succeeded his father as King. Nonetheless, she bore Valacar an heir. He was raised with the name Vinitharya in the eastern lands, but returned to Gondor with his father as Eldacar. These events caused unrest throughout Valacar's reign - it was believed that the pure Númenórean blood of the Royal House would be lost.
By the last years of Valacar's reign, certain southern provinces of Gondor saw this unrest break out into full rebellion. In the year Valacar died, III 1432, this rebellion became a civil war, a dark and bloody period known as the Kin-strife that would last for the next fifteen years, until Eldacar managed to secure his throne.
Valacar ruled as King of Gondor for sixty-six years, and was succeeded through great troubles by his son Eldacar.
Notes
1
The date of Valacar'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
Valacar's name apparently reflects dedication to the Valar, as the name of his son Eldacar ('Elf helm') represented a comparable relationship to the Elves. Just as Eldacar had an Anglo-Saxon equivalent in Elfhelm, there was also an Old English name with a meaning comparable to Valacar. Unlike Elfhelm that Old English name is not recorded among the Rohirrim, but it would be related to the name usually spelt Anselm today.
3
According to a note reproduced in volume XII of The History of Middle-earth, Valacar and Vidumavi had 'children' - that is, at least one other child beside Eldacar. Nothing more is said of them, except that they inherited their mother's considerable natural courage.