Shackles won't help. They only prevent Nicholas from fleeing the battlefield at a low difficulty level. They are completely useless on the hero himself, as even without them, the enemy never runs away. They just take up a slot. And you can't transfer them to another hero.
In any case, if Nicholas doesn't flee the battlefield, he will still reappear somewhere.
P.S. Dragons near the prophet may have a different artifact instead of shackles.
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