My troupe-mates have an interesting idea. They found a place with a pile of ghosts in it. They want to clear the place out, but want to minimize the risk. Here is the idea.

You cast an Aegis around the ghost infected castle. What are the effects??

1) I believe the ghosts (or any might creature) is not expelled.

2) All might creatures inside have their casting totals reduced by 1/2 of the aegis. Their might score for purposes of them being dispelled remains the same. It costs the might creatures the same amount of might points to "do" their effect: just the penetration is lowered. Can the might creature increase the number of might points they spend to "get over" the effect of the aegis?

3) How would you handle the ghosts knowing what the mages are doing and possibly interfering with the aegis ritual? Why would they know what the mages were doing?

The Aegis is a ritual spell that should take a couple of hours to cast, with the magi chanting and marching round the boundary. The ghosts might not know *what* the magi are doing (unless any of the ghosts are former magi) but they would definitely know the magi were casting some kind of major spell, and that can't be good for the ghosts. I think they would try to disrupt the casting any way they can, forcing the magi to make Concentration rolls. And if all the casters fail their rolls, the vis is wasted!

I do not think the ghosts would be able to spend extra Might in order to boost their Penetration. In fact, if they spent more Might their Penetration would go down if you're playing Fifth Edition (p. 184). But for the most part supernatural creatures cannot alter the might cost of their powers - they just pay one flat rate.

What this means to me is that the ghosts would still be around and their Penetration would be reduced to the point where they have a hard time affecting the magi. What it doesn't mean is that the magi can just ignore the ghosts. While the ghosts cannot affect the magi directly, they can still do things like haunt the place making annoying moaning sounds, scare off the grogs, maybe move objects around or even knock stones off the ruined castle so they fall and hurt people, etc. Casting Aegis may make it safer to enter the ruins but it would not solve the ghost problem by a long shot in my opinion.