i'm going to talk about the Keep level for the Master of the Hunt for SoB, so if you don't want to know about it, go away.
It seems like this could go very badly for the heroes very quickly if they have a bad first turn. All the Master has to do is run past them and turn sideways, and the heroes are stuck until they chew through his several hundred wounds. Then the wild hunt catches up and the overlord wins. I think using the master to prevent the heroes from fleeing the wild hunt might fall under the "creating an unwinnable blockage" rule - it just seems like a cheesy tactic that cheapens the end fight. I am assuming that the hellhounds representing the hunt are just markers, that the heroes can't actually just kill the hunt. Perhaps allowing the heroes to also move through the Master would fix this problem.