Three-Headed Serpent

The origins of the god known as the Three-Headed Serpent are uncertain. Perhaps they fell victim to the conquering Dark Lords, or to one of the last of the Colossi. However it happened, they were killed, in as much as gods can be killed; robbed of material form, stripped of most of their power and cast into the Abyss. There, their discorporate essences festered in rage and resentment, before they were able to infiltrate the dreams of a group of spirit priests.
They came to the priests of a fledgling Human kingdom, beset on two sides by powerful enemies. They offered power, and their offer was accepted. They took the worship that the priests offered, and soon moved on to demanding blood sacrifices. These gave them power, a vestige of their old way of being, and a hunger for more of the same. They assumed the serpentine forms of the spirits that the priests had previously appealed to, twisting their evil and malicious natures into writhing, scaled form. They demanded more and greater sacrifices, and in return taught the priests to transform themselves and their chiefs into monstrous echoes of their own aspect.
Perhaps they were always three. Perhaps the nature of their worship made three from more, or fewer. Whatever the case, these once-dead gods became the dread triad of Stygis the Fearmonger, Asphyx the Slayer and Sythis the Betrayer, gods of the Yuan-ti and patrons of the Ophionic Empire.