The Fortress of Zhon
This particular beast lay dormant for some time. If it was birthed in the area then surely it is responsible for the wanton destruction of the surrounding area.
It is said that this particular demon hunts in packs and will summon its brood brother should anything go awry.
The only weakness you found was that its hide was immensely vulnerable to the magical energy of the nearby Elven Crystal that surrounded the tome you were after.