The fifth King to rule over the North-kingdom, son of Eldacar, grandson of Valandil and great-grandson of Isildur himself. Though his reign spanned nearly a century, we have no records of events that took place in that time. He was succeeded as King by his son Tarcil.
The date of Arantar's birth appears only in The History of Middle-earth volume 12, The Peoples of Middle-earth. It cannot therefore be considered completely reliable.
Arantar was not in fact the High King; no ruler had borne that title since his great-grandfather Isildur. The choice of name is interesting, and may imply that Arantar (or perhaps his father Eldacar) was making some kind of claim to rule both the Kingdoms of the Dúnedain. No historical record of such a claim exists.