Amlaith of Fornost was the eldest son of Eärendur, the tenth King of Arnor.
He was destined to become King of Arnor after his father's death in Third Age 861, but there were constant quarrels between Eärendur's sons.
A civil war erupted when Eärendur finally died. Amlaith, however, failed to overcome his two brothers and founded the kingdom of Arthedain. (His brothers founded the kingdoms of Cardolan and Rhudaur).
His reign in Arthedain was largely a peaceful one, although border disputes with the realms of Cardolan and Rhudaur never fully ceased. Under his rule the capital was formally removed from Annúminas to Fornost.
Unlike his forefathers, he did not take a name in Quenya the "High Tongue", but instead took a name in Sindarin.
He was succeeded at his death in 946 by his son Beleg.
The name Amlaith is difficult to translate, but may mean "Uprising Laughter" in Sindarin (from am = "up, upwards" and lalaith = "laughter").