Well it won't work like that, this anti-teleporting spell will not hold him forever so you kill him in battle or he'll run.
You have nothing with which you could pressure him for information. If you haven't noticed yet he actually enjoys pain and death won't make him cringe either. He's not talkative enough to spoil secrets just because he wants to scare you or anything.
You can keep his helmet and sword and if you're really paranoid keep his hand, he won't be back though:
Here are the results:
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