Amlaith of Fornost (TA 726 - TA 946) was the eldest son of Eärendur of Arnor and hence a direct descendant of Isildur, Amlaith should by right have succeeded his father to become the eleventh King of Arnor and lord of all the Dúnedain of the North. After Eärendur's death, however, Amlaith fell into dispute with his younger brothers, and the descent of the Kingship was interrupted.

The details of the brothers' disagreements are nowhere recorded, but their result was of historical significance, causing the old North-kingdom of the Dúnedain to be split into three realms. Amlaith's realm was reduced to the lands west of the River Baranduin and the Weather Hills, a region that came to be known as Arthedain. The eastern lands of Arnor were divided between Amlaith's brothers, creating the new kingdoms of Rhudaur to the north and Cardolan to the south.

Whatever the cause of the dissension between Amlaith and his brothers, the fact of his direct descent from Isildur was never in question. Through Amlaith, the Kings of Arthedain carried the bloodline of Elendil down through the generations.

Unlike his forefathers, he did not take a name in Quenya the High Tongue, but instead took a name in Sindarin.

He ruled for eighty-five years and was succeeded at his death in TA 946 by his son Beleg.