MuIm 40 - Voice | Conc | Vision | - Hallucinations
The caster can henceforth change what the targeted individual sees, smells, feels and hears. This can be quite disturbing as the environment does not actually change but what the victim perceives changes. The caster can make the victim see a bridge where there is none or bring about a virtual hellhound that chases the victim so long as the caster remains in range of the target. The dog cannot bite and fire will not burn but it will all seem real up to that point to the victim.
I don't think this works as a MuIm spell. The MuIm benchmarks affect 'an object', not a person. Affecting a person would presumably be a Me spell, but I don't see a good Me benchmark that does what you're after. Maybe MuMe 15 (completely change a person's mind), with a Re requisite to control what the mind is experiencing?
This feels more like a Muto Mentem effect to me, especially if the goal is to change how a person perceives the object rather than change how the object is perceived (if you see the difference).
The MuMe guidelines say that MuMe can be used to change "emotions and thoughts that people cannot normally feel." I don't think that it would be hard to argue that making you see something that isn't there, or vice versa, would be akin to a thought that one cannot normally feel.
I would call it a Base 1 (equivalent to a "minor change in a person's memory of an event") to make them see, or here, or feel something that isn't there, if it only affects one sense (to see the non-existent bridge). Base 2 would be for multiple senses (for example, to see and hear the hellhound).
So the spell level would be based on how flexible you want it to be. Call it Base 2 to be on the safe side. R: Voice, D: Concentration, T: Individual makes the spell level 5...and surprisingly cheap, unless I'm way off base somewhere here.
I highly recommend looking at p.65-68 of HoH:S. These pages clarify when to use Imaginem and when to use Mentem. They also clarify when to use which Techniques. Your description sounds a lot like a blend of the two methods for attacking a mind's capacity to interpret what the senses perceive. While it could certainly be argued that you only need to use MuMe to force the mind to misinterpret things, based on the examples provided and the descriptions of what happens that would probably require an exceedingly high Finesse score. However, targets whose senses are weakened in some way (drunk, tired, etc.) are easier to affect. So I would think Mu(Pe)Me would be the ideal way to go. MuMe changes what the target perceives. PeMe reduces the target's mental capacity to interpret, making the target more susceptible. Now your target is very likely to believe the craziness presented via the spell.
I would disagree on the base. This is not a slight misinterpretation of what was seen. Rather, it's complete control over what is observed. It also affects all the senses, not just one or even two. To me this would be more akin to "Completely rewrite a person's memories" (base 10). I could see an argument of a major change to one sense being base 2 (like your base 1), and then adding a magnitude for each additional sense as does Imaginem. This would also bring it to an effective base of 10. So I see it as:
Then, as I mentioned above, I could see adding a Perdo requisite to make the target more susceptible to make sure the illusion is truly believed, which would make it Mu(Pe)Me30. This isn't needed, but it can very helpful by preventing the target from realizing all the inconsistencies that exist.
Hmmm... I thought my reasoning was pretty good before, but reading another reference makes me think Cr(Pe)Me is the right way to go. Take a look at the CrMe25 spell Human Figment of the Waking Mind (A&A p.32). This is essentially a low-level equivalent of what you want. It's CrMe because a nonexistent hallucination is being placed in the person's mind. It follows putting words in another's mind without limiting it to the audible. Yes, it does add +1 per extra sense. It also adds magnitudes for control and intricacy. I'd add the Perdo requisite to make sure the target isn't paying attention to the real sensory input, just the false stuff.