Goremaw the Devourer
Goremaw the Devourer was a massive felhound and the personal hound of the Dark Titan, Sargeras. Sargeras must have cared deeply for his pet, for he sculpted its power, gifting its sharp fangs with enough shadow energy to pierce reality itself. One of the beast's most notable features was a potent withering essence that made its bites extremely deadly.
When the Burning Legion invaded a new world, Goremaw would often accompany them, tearing enemy soldiers to shreds. Between battles, the felhound's eredar caretakers would collect the shadow energy dripping from its fangs, using it to craft powerful new weapons and reagents.
One such world had already been pacified, its leaders corrupted and its heroes slain. However, a lone child crept into the Legion's stronghold and killed several eredar and even Goremaw itself while they rested. When Sargeras saw his slain hound, he was consumed with rage and annihilated the world, scattering it as ashes in the Great Dark.
A dreadlord named Mephistroth believed that Goremaw could still serve the Legion. Extracting the beast's fangs, he took them to a demon forge on the world of Argus to reforge them into a pair of deadly daggers. Though he dared not handle the task himself, his servants were eager to distinguish themselves. However, the fangs' twisted power often proved too dangerous, piercing their minds and inflicting unspeakable agony. When this happened, Mephistroth simply gave their souls to the forge and found another servant to take their place. In the end, the effort was successful, and Goremaw's teeth had become the Fangs of the Devourer, their shadowy power and deadly withering essence preserved for all time. The daggers were eventually passed to Akaari Shadowgore, one of Sargeras' deadliest assassins.
Sargeras' own hound was maybe once a felstalker, but over the years the Dark Titan reforged and reshaped his companion into something much more powerful. When, at last, he was pleased with his creation, he set about creating more of the hounds, and imbuing them with shadow and flame. F'harg and Shatug are such examples.