Ouboroth's downfall was orchestrated by Sekhar, a servant of Neferata and Nagash, when the Vampire infiltrated the god-serpent's priesthood, undermining them over time. Eventually Nagash was able to destroy Oubroth. A much diminished avatar is all that remains of the deific snake, which Nagash gifted to Sekhar both as a reward and so the vampire could use her talents, hypnosis and spells to subvert the mind, to keep the snake placid.
Ever since Sekhar and this husk of Ouboroth have fought alongside one another. On occasion Sekhar allows the ever-hungering husk to devour souls, doing so is a great risk, for the power that doing so brings.