No, that's not very interesting.
You get on a ship (mermaids, +2 obelisks), the demon circles around the portal - you get on - and that's it, you're done, no matter where you sail, the demon runs near the portal. I'll teleport a little further with a cheat, otherwise it's not right, the hero has things to do there, but he can't go. If it would at least run away a little further. That is, you can't just get off the ship honestly. (Ideally, there should be another point where you can get off, or a one-way portal).