Aranarth (S. 'King of the Realm'; aran = king, arth = compare, arda = realm) was the first of the Chieftains of the Dúnedain, and would have been King of Arnor at the death of his father in T.A. 1975. He was the son of Arvedui, last King of the North Kingdom.
When Aranarth was still a youth by the standards of his people, the Witch-king of Angmar destroyed the Northern Kingdom, overrunning Fornost. Most of the people, including Aranarth, fled to Lindon, but the King Arvedui went north to the Icebay of Forochel.
By right, this made Aranarth now King of Arnor, but since his Kingdom had been destroyed he did not claim the title. Aranarth rode with the army of Gondor under Eärnur and saw the destruction of Angmar.
While the Rangers defended Arnor from the remnants of Angmar's evil, the Wizard Gandalf went to Dol Guldur, and drove out Sauron the Necromancer. Thus began the period known as the Watchful Peace, a time where attacks by the enemy were few and far between.