Charisma and Magic

I’m really not a rules hacker, and am conservative with any tweaks I introduce, but something ChicagoWiz posted the other day got me thinking. Among other things, he proposed a “What if” where magic users were sorcerers and relied on charisma as well as intelligence. It’s a relatively neglected attribute that I’ve always wanted to give more attention. I really don’t want to overhaul the class, but the notion that charisma is taken into consideration with certain spells makes a lot of sense and makes me want to introduce it in my Swords & Wizardry games somehow.

Here’s a list of spells from the core rules that I’m considering combining with charisma in some manner:

MU: Charm spells, Monster Summoning, Suggestion, Conjure Elemental, Geas, Power Words
Cleric: Bless, Speak with (Animals, Dead, Monsters), Quest, Aerial Servant, Holy Word

The first thing that comes to mind is to simply use the typical attribute modifier with any saving throw a monster needs to make. This would be very modest - a +1/-1 thing. In the case of NPCs though, the difference in the spell caster and target’s charisma might be more fun, but it might also dramatically over or under-power the spell. That is, a 16 charisma spell caster casts Geas against a 10 charisma NPC. Forcing the NPC to save at -6 seems harsh. Maybe halving this and rounding makes sense, or I could always scale it somehow.

At first glance, it seems like a minor house rule. It’s a small group of spells where this would be used. I already use the 3d6 in order method for determining attributes, with no rearranging or re-rolls, so it would make overpowering spell casters less likely.

On the other hand, using charisma like this might add some extra flavor into the game and game balance be damned. It makes sense that a highly charismatic magic user would acquire and be predisposed to use spells that depended upon his personal magnetism, just as an ugly witch would have a harder than usual time in charming the rare visitor.

I think this is something that’s used to a degree in 3rd edition, but I’ve never played it and am not familiar with the rules. Has anyone done something like this (or more ambitious) with their own house rules in an earlier game version?