Bringing Cerebromancy Back : Charm Spells.

Proper Definition of Charm spells

Charm spells increase the target’s regard and affection for the caster. The magic is not fundamentally about getting the target to DO things for the caster, the magic is about getting the target to FEEL a certain way about the caster. This makes persuasion easier, but that is not the essence of the spell.

"Cast a spell to make the target Do This Thing" is a different line of spells, a different branch of cerebromancy. (You can obviously use charm spells to help your nonmagical persuasion, just as you can use Charisma-boosting spells to help. But it's not a direct effect of the spell).

Charm spells create a false affection for the caster.  Least charm creates the feeling that the caster is, for some reason, too special to be directly harmed.  True charms (minor charm, major charm, supreme charm) create the feeling that the caster is a friend and ally.  If the caster is in combat with the target's allies, it must be the result of some misunderstanding, and the target must act to protect the caster and try to stop the fighting.  

Quick Rules for Social Interactions

Out of combat, making charm spells useful in-game means having a robust structure for social interactions in the first place.  Rich Burlew, Giant In The Playground, author of Order Of The Stick did a pretty good fix to the 3.5 diplomacy rules, which I've tweaked.   

You roll a Charisma / Persuasion / Diplomacy check, modified by two big factors:  how attractive is what you're saying (+10 / + 5 / +0 / -5 / -10), and how does the target feel about you (+10 / + 5 / +0 / -5 / -10).  Base DC is 15 + target's level/HD + Wisdom modifier. (If the target has advisors in range, use their Wisdom instead.)  The numbers are scaled to 3X, but I think the principles can be used in any D&D system.  

With that system in place, it's much easier to decide what charm spells actually can and cannot do. 

The Spells

0.  Least Charm.  On a failed save, for 1 minute, target is unable to attack caster, regards caster as no worse than Neutral (does not treat caster as enemy).  When the spell ends, target remembers the spell being cast.  

1.  Minor Charm.  On a failed save, for 1 minute, target regards caster as a friend / ally.  Will not attack caster, will take risks to protect caster.  If the caster is in combat with the target's allies, it must be the result of some misunderstanding, and the target must act to protect the caster and try to stop the fighting.  When the spell ends, target remembers the spell being cast, and may break any agreements made.  

2.  Major Charm.  As Minor Charm, for a 10 minute duration and target does not remember the spell being cast.  All things being equal, target will keep agreements made.  

3.  Supreme Charm.  As Minor Charm, but permanent until dispelled.  Target remembers the spell being cast.  Target is not likely to keep agreements made when the caster is no longer present to enforce their will.  

3.5's Epic Skills provide a framework for more powerful Charm effects, the "Fanatic" column on the reaction table.  But I'm not a fan--if the spell gets cast on a PC, you're basically an DM-controlled NPC while the spell is active.  That's bad gaming, which makes is bad design.  "How do you stop your good friends The Band of Benevolent Brothers from fighting my other good friend Bane Blackheart" is interesting and fun.  "You love Bane Blackheart now, you need to attack your friends for him.  Make your attack roll." is not. 

In Play.  

This gives charm spells obvious combat utility.  Having an enemy refuse to attack one PC for an entire combat certainly changes the combat.  Having an enemy or a PC decide that the two sides should not be fighting, and actively impede the fight, creates fun and interesting options.  Especially if you have mechanical support for Big Strong Fighter types doing things besides Hit For Damage--spell targets are likely to tank attacks on the caster, interpose between the caster and hostiles, grapple allies trying to attack the caster.  

One thing I have not resolved is--could a charmed spellcaster target the caster AND her allies with spells to make them stop fighting?  Calm emotions, mass command "drop", etc?  It fits with the spirit of the spell--treating the caster as a friend and ally, the same as the target's actual friends and allies.  But is it too open to munchkinry and abuse? 

And having a rubric to evaluate what the player wants to munchkin the NPCs out of makes charm spells game-able again--neither complete "I win" buttons, nor nerfed into uselessness to prevent Charm Person from letting the caster run roughshod over the setting.