Confusingly unrelated to the creation of charms, (mostly) Charm magic is a generic term for those types of magic which control and manipulate the target's mind and/or take over control of their body. These can overlap with illusion magic given that traditional illusions are sometimes said to be at least partially in the victim's head1.
This is not often played as sinister as it should be - when people think of black magic, charms are generally not on the list but it doesn't take much analysis to work out that mind control can and should be a lot scarier. A necromancer only uses your body after you are done with it (unless your setting has the undead powered by enslaved souls) - someone who works a charm on you does it whilst you are still using it. The potential abuses of charm magic range from simple fraud to mind rape to effectively re-writing someone's entirely personality to dragging them along as a passenger in their own body. In fact, it's hard to see any applications of charm magic that aren't abusive except for clearing up after all the others.
Obviously, this is about as direct as magical attacks get and any resistance mechanisms in the system should apply to it … but so should the laws of magic surrounding consent and suchlike. Not to mention humbugs. Leaving a backdoor through your defences for this sort of thing is a common side effect of witch boons and other similarly bad bargains.
This is, of course, great for values dissonance with the pacifist wizard arguing that he is not going around killing people and is actually reforming them and allowing them to live happy and productive lives in the service of good … whilst his enemies suggest that what he's doing is worse - they may kill, but at least they don't force people to like it.
It's also the school of magic which creates plot driving things such as the Celtic geas or tynged - curse like obligations or prohibitions - but these traditionally originate from fate or from other powers with the moral authority to deploy them. Their fRPG descendants, of course, have been the bane of many players over the years. More mechanistically, a geas may oblige a victim to answer questions - or prevent them from doing so … conflicting geasi may cancel one another out, or not - with less fortunate victims obliged to suffer the negative consequences of one geas whatever they do2.
Prolonged use of mind control is liable to either rob a subject of their motivation (and/or personality) or, subject to the accumulation of enough mutually contradictory instructions, leave them brainwashed and crazy.
Game and Story Use
- Players will hate their characters being the target of charm magic - even if those characters have used it quite promiscuously themselves. Bring it into the campaign a lot and they'll likely bail in disgust at the lack of agency. Use it rarely and give them ready made hate against the villain responsible.
- That said, quite a lot of old school fantasy uses geasi to drive the plot - specific characters are magically compelled to undertake various tasks, sometimes as a result of angering a god, sometimes as a judicial punishment and sometimes as part of a bargain they've made. Players might accept a geas in exchange for averting character death…
- Some more thoughtful settings explicitly forbid this sort of magic - using it is a capital crime in the Dresedenverse, whilst the Potterverse is nightmarishly inconsistent: one specific application is outlawed whilst others seem to be common practice. Badly considered settings (such as certain fRPGs) treat it as morally neutral…
- In the GrimDark of Stross' Laundryverse ubiquitous geasing seems to be accepted as a necessary evil - albeit one that is recognised to be easy to overuse.
- As far as the moral dissonance goes, you could have a spectrum of different kinds of charm magic. At one end are things like rewriting personalities; while the other simply enhances the caster's natural persuasiveness.
- A few possible (semi-)beneficial uses for this sort of thing might be: