Bul-Kathos is one of the original Nephalem, one of great power who guards Mount Arreat, the site of the Worldstone, alongside his children. The Barbarians worship Bul-Kathos as a deity, he is their king and their ruler. 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.
Appearance
Quote below taken from the second novel of the Sin War Trilogy, Scales of the Serpant. One must keep in mind the time at which the novel occurs to get a true idea of his original appearance, and also his age.
"The giant moved toward Uldyssian, and as he did, more stone and dirt fell away. Now, although he still looked as if created from the very ground, Bul-Kathos wore the vague semblance of a bearded warrior. His skin was the brown of the soil and his hair the green of grass"
"More and more the earthen creature gave way to an ancient warrior. Clad in kilt and sandals and with a golden band around his head, Bul-Kathos resembled some Barbarian deity"
"Bul-Kathos existed more as spirit; his true body had long ago been replaced by the soil in which he had lain."