Bul-Kathos
Bul-Kathos was a great and ancient Barbarian king, who organised the tribes to the modern day people who call themselves the Children of Bul-Kathos.
Background
Bul-Kathos is mostly known to have united the barbarian tribes, and set the people upon the great "vigil" of protecting Mount arreat, and the Worldstone within. While it's unknown if even the Barbarians knew that the Worldstone was inside, or what sort of power it had, Bul-Kathos is the most revered barbarian through time, and the barbarian people call themselves the Children of Bul-Kathos in his honour.
Men in the Western Kingdoms gave the tribes the epitet "Barbarians", as they only saw the grim outlook of the people, but at heart, they are a honourable people, with Bul-Kathos deeds as guide.
Bul-Kathos knew the secrets of the ancients, the mysteries of what's below Mount Arreat and of the prophecies regarding the dark times ahead. He shared these secrets with a close friend (or possibly brother) called Fiacla-Géar ("Sharp-Tooth"). When the two started to devote themselves and their people to protecting the secrets, they disagreed on exactly how to do this.
They both agreed the other's philosophy had merit, so when Bul-Kathos united the tribes and training them in strict martial discipline to make sure the tribes faithfully would concentrate on their goal for generations to come, Fiacla-Géar gathered together a small group consisting of the tribes' greatest warrior-poets and shamans and mysteriously retreated into the forests surrounding the area known as Scosglen. The Druids were henceforth separated from the barbarians, fighting the same war in different ways.
The only currently known tome to have this information is the ancient tome of the Druids, Scéal Fada ("Long Story").