What a shame. Whatever Umbranse has done, he has prevented me from undoing his magic.
He is wiser than I had anticipated.
I do have a backup plan, however. I have seen owlbeasts enter and leave Mazthoril, even after Umbranse took control. Their ancient druidic magic must be capable of disabling Umbranse's. Luckily, I happen to be somewhat of an expert in their methods.
Travel south, to Dun Mandarr. You will find a chest with a dreamcatcher in it.