He is described in the second book "Scales of the Serpent" of the Sin War Trilogy written by Richard A. Knaak, about the mythical Bul-Kathos.
This description has been taken from page 121 of the second book of the Sin War Trilogy "Clad in kilt and sandals and with a golden band around his head, Bul-Kathos resembled some barbarian deity".
It is also described in Richard A. Knaak's book that Bul-Kathos was the guardian of Mount Arreat itself.
...the two combatants slowly circled each other in the middle of a gigantic mob of shouting, fur clad giants, every muscle in their body swollen to impossible size, staring at each other with hatred burning fierily in their eyes. This was a battle of life and death, deciding which of these two warriors would have the honor of commanding the barbaric tribes of the north into battle. The current champion, battlescared from countless victorious encounters, was wielding the famed relics of the tribes, the legendary twin swords called the Children of Bul Kathos. Facing him off was a younger challenger, an unknown warrior that had acquired a great treasure while adventuring across the lands of Sanctuary. This treasure was the The Grandfather and Doombringer, two mythical swords imbued with powerful magiks. The legends said that these swords were lost forever in a titanic battle of good and evil, where the champions of both sides annihilated themselves and their armies by crossing these blades in mortal combat. The veteran tensed, ready to jump forward while the younger warrior seeing this lifted the twin swords in front of him, crossing their blades. An explosion of magical sparks and light blinded the surrounding crowd as the combatants clashed their blades, screaming and howling and chanting barbaric battle hymns.
"Steeped in mystery and tradition, these people refer to themselves as the "Children of Bul-Kathos", the great and ancient king. To better protect their lands from outside forces, they adopted a nomadic lifestyle, frequently moving within the confines of the Steppes and maintaining few permanent settlements. Isolating themselves from the world outside their territories, they eschew the use of magics and complex machinery, as they believe these things can only weaken their resolve developed over so many years." "The Children of Bul-Kathos have developed a kinship with the land and have learned to harness the primal energies within nature to enhance their own substantial physical prowess. It is because of this, together with their freedom from the trappings of the outside world, that the Western Kingdoms have historically referred to these tribes as "Barbarians", an epithet that belies the rich cultural and spiritual history these people truly possess. Although some trade with these curious bands of nomads, they do so only along the outermost borders of their lands. All intrusions into the territory surrounding Mount Arreat are forbidden, and warriors from the northern tribes are quick to thwart any incursions. Every foreign attempt at conquest has been met with fierce and decisive resistance.''