Bul-Kathos is one of the original nephalem. After Lilith murdered all angels and demons on Sanctuary, including his parents, several nephalem tried to seize the power of the Worldstone for themselves. Bul-Kathos and other nephalem fought to protect the Worldstone. They succeeded in stopping them, and vowed to continue to protect Mount Arreat. As the centuries passed the nephalem set to stand guard over the mountain all died, with the exception of Bul-Kathos who became one with the earth where he slept.
His only known family is Vasily, his brother.
Bul-Kathos is a huge warrior of great strength. As he slept he eventually turned into the same earth he slept in. He can take the apperance of a normal nephalem, but he doesn't appear to be of flesh and blood any more.
During the Sin WarEdit
Bul-Kathos is awakened from his long sleep by Uldyssian when the human appears near Mount Arreat. Bul-Kathos warns him to stay away from the mountain, and challenges the human when he tries to pass by the old nephalem. The two warriors are evenly matched and it is only by the intervention of Rathma that the fight ceases, after which Uldyssian and Rathma head on towards the Worldstone.
Bul-Kathos also reappears during the final battle between the edyrem and Inarius, and the subsequent battle involving Heaven and Hell near the end of The Veiled Prophet. it is unclear whether he survives the battle, and, therefore, whether he still lives.
The Barbarians worship Bul-Kathos as a deity, he is their king and their ruler. He and Fiacla-Géar were leaders of their people and tasked with protecting Sanctuary. They disagreed on how best to do it however. Fiacla-Géar thus split of with some of their people to become the Druids, while Bul-Kathos formed the barbarians.
Although there are many gifts from the Great and Ancient King Bul-Kathos, the greatest of these is the secret of steel. Raw iron is hardened and made resilient, forged into weapons of honor and power. Most mortal swords are patterned after the blade first wielded by Bul-Kathos himself. Balancing offense with defense, it is the perfect weapon for the defense of Mount Arreat in the coming apocalypse. All Barbarian warriors learn the secrets of steel at an early age, yet few truly master the deadly elegance of the sword. Those few who do often disdain all other weapons.