One of my player recently asked me what would happen if he casts a mind control spell on a target who is already under the influence of a mind control spell.
I admit I have absolutely no idea how to handle this; I can't logically take the Penetration in account, as there's no magic resistance. I could assume that the mightiest spell overcome the weakest, but what to do in case of a tie?
I would say it would be the stronger of the two spells that would win out, not necessarily the most recent. I would personally run it as a comparison or "fight" between the two effects by probably giving the one which has more of the total devoted to penetration the edge ([spell 1] penetration before die roll + SD) vs the other guy's spell [spell 2] also rated for strength by the pre-roll penetration total + SD).
Whoever gets higher gets control or maintains it. Hermetico-psychic tug of war.
"The battle for our minds is enjoined, woe betide us when they take aim upon our hearts."
Well, since penetration these days describes how well you get past defenses, and level is the power of a magical effect, I'd say that level is what matters and penetration is unimportant.
There are two important points here though. The first is that by level, I mean the base level (+ any additional magnitudes dedicated to power and making things harder to resist [cf. adding magnitudes to invisibility spells to make them harder to pierce]) rather than the total level. The extra levels spent on range go towards increasing the range, likewise duration.
Secondly, unless the two effects directly compete, why not have them coincide? A command to avoid violence and a command to kill someone don't ever need to come into conflict unless the target is encountered.
I, for the most part, agree with Ftagn. I suspect that most mentem spells aren't going to be mutually incompatible, let them both function normally and only go to some sort of a tug of war system if they are pulling the mind in a way that doesn't permit both spells to work properly.
If you do end up comparing spells I think that the spell with the higher base level should win (if the base level is the same I'd use penetration as a tie breaker).
If you happen to have access to Art and academe there is an enlightening section on the human mind that shows the mind's different parts and method of functioning thereby giving the mentem magus a description of the ways that the mind can be altered.
If the Duration of the original spell has lapsed, then the new one takes effect.
Assuming that the original spell is of sufficient duration that it is still on-going, then if the spells are not contradictory, then I'd let both take effect.
If the spells are contradictory, then I think that I would just rule that the highest magnitude one takes precedence.
If the magnitudes are the same then, I'd rule that the original spell takes precedence, and possibly (if I was feeling evil) that the second caster rolls botch die, just as if he had rolled a zero on the casting die.
The reason that I wouldn't use Penetration to decide (even a tie) is just that Penetration is not something I would routinely keep track of. So, I'd have no guarantee that I'd know the Penetration of the original spell anymore, and I'd personally rather not use house rules that require additional record keeping. I don't want to be in the position that I need to keep track of the Penetration of every mind-altering spell just in case another spell is cast on the same target some time in the future.
IMS we have a Bjornaer Mentem specialist, with a focus in Domination.
So this is interesting.
I'd say, than as long as the effects aren't contradictory, they'd all be in effect. In the case of contradiction, the higher level of effect should take precedence. Since penetration merely helps overcome magical/supernatural defence, the greater a spell is, the more likely to win the mental, internal battle.
Also, I'd have personality traits modify this heavily, and also the things that trigger either effect. A person being forced into something his personality opposes, could fight back. Outside events might have magically induced tendencies flare up again, even if the person fights it, or another mind control spell tries to take him in a different direction. If the magus who'd cast the spell shouts his order again, if the subject of the burning hate reappears etc.
Finally, for the purpuses of simplicity, it should be spell level, because - as it was mentioned before - penetration might not be noted. And if you use items, the same lab total will make a much higher level of effect, if you design without penetration. So the item made for control of mundanes is more specialized, and should take precedence in a conflict of mentem spells, compared to an item of same total level, with added penetration. Because this item with penetration is more flexible, as it can also be used on magi.
On a related topic, I mentioned Personality Traits affecting mentem spells. How do people handle this? If the magic is particularly strong, it'd override any natural instincts or what? Is this covered by the definitions of "natural emotions" vs. "unnatural emotions"? If the peasant is a coward, forcing him to stab his knife in the noble, face to face, among his armed bodyguards, would be unnatural when compared to his Bravery. In the back, with the nobleman all alone in the stables, that'd be within the scope even of a coward. But his Loyalty or aversion to killing from his christian faith might prevent him.
OTOH a trained soldier might be brave or battlehardened enough to go in clinch with the noble and his bodyguards, making it less unnatural.