I don't understand the cloud either.
I see what you mean by the ghost. As 'ghost' was the working name for the Seep, let's call it something else for clarification, like Boo. (Also, Erik wants to keep the supernatural out of the Eighth for flavour purposes.)
Anyway, how the boo works is that it moves towards you like a roach until you look at it (directly or line-of-sight, I'm not sure. Directly makes the monster mostly trivial, line-of-sight more difficult computationally but more effective). When you do, it disappears, like the Boos in Mario games. Unlike spiders, which also disappear, the Boos are harmless when you're looking at them and won't move. If a monster crosses the square in which a harmless Boo is in, the Boo takes over the monster, which moves 'chaotically' for ten turns. How the monster will move when moving 'chaotically' I don't know. Killing the monster in this state will kill both the monster and the Boo.
The key to the Boo, I think, is coming up with interesting 'chaotic' movement behaviour and having that be the easiest way to kill the monster.
I believe the duplicator has been suggested before. I'd definitely class it as a B-list suggestion, though - it's a good suggestion, but there are lots of things I'd give it up for.
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