first of all, to remove him from the invulnerable state, you have to kill all the other creatures in the room , especially those around him doing the rituals, if not you will NEVER be able to inflict any damage on him.
though he may appear to be impossible to defeat, but once you can inflict a certain amount of damage onto him, he will surrender and be willing to go back wif you. so keep whacking ... =)
if you have any potions like cat's grace or bull's strength, you may wanna consider using them.
=)
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