I wanted to make a necromancer, and I find the idea of being able to physically threaten (rather than indirectly threaten through, say, saying you'll desecrate their grave) spirits into servitude to be, simply put, freaking awesome. However, to my greatest disappointment, neither Rego nor Mentem have anything to offer as far as banishing or harming spirits. In fact, given the usually-incorporeal nature of spirits, there really isn't much I can give this magus to threaten spirits with except for that Perdo Vim Might-draining guideline. Not a banish option in sight at all, and... Well, Perdo Vim.
I suppose it's not totally unreasonable that Mentem can be combined with Rego to control spirits, but not Perdo to destroy them. Banishment of spirits seems like it would be both reasonable and even still Rego Mentem, but well, the guideline doesn't exist except maybe as an extrapolation of "control a disembodied spirit" which, given that it has to carry out the action itself, will probably amount to no more than it getting off your property so to speak.
I'll quit beating around the bush. Can you guys think of anything, without straying into pure houserules but perhaps including extended interpretations of things that exist in the rules, that would allow the character to have a threat against spirits he can truly back up that doesn't require him to gain skill in an entirely separate TeFo or anything else equally xp-taxing? The closest possibility I've seen is that Banishing ability from HoH:Societates, and that requires the banishment of all spirits from an area and is still something of a "get off my lawn you pesky kids" while I'd like something that leaves a more lasting (if at least psychological) impression and can be targeted more selectively... Like, say, a spell, though that option doesn't work the way I'm trying to make it work.
EDIT: I forgot to mention while writing this lengthy rant: I'm totally open to trying other types of threats if you think they'd work better; for example, if it would work, maybe I could try threatening to drive a given spirit insane or deny its ability to feed on whatever it sustains itself on. If you have ideas in the vain of "change your threats" please feel free to suggest them, I could use any help.
Ermm... just checking, but...
You did read Lay to Rest the Haunting Spirit (ArM5, p. 150, top of column 3), right?
Because to me it looks like the spell you're asking for. If it isn't, I'm going to need a little more detail I fear.
So I feel like an idiot now. My copy of the rulebook pdf on my computer is entirely missing page 150, it would seem. But upon inspection of my physical book, you're absolutely right, the spell I desire is right there. Sorry for wasting your time.
EDIT: No it's not missing. I literally just didn't see that spell. I'm the silliest human on this earth.