Question: I've been working on a Pathfinder campaign. The backstory of the homebrew campaign I'm running implies that some characters are born with an amount of magical strength that sticks with them throughout their life. If someone were to be a powerful mage, it would be due to the fortune of their birth.

My idea was that I could add another Ability Score (Magic) and use that as a substitute for the modifier on rolls related to magic instead of things like Charisma, Intelligence, or Wisdom.

Would this break the game? Looking for feedback on this change from people more experienced with the Pathfinder system. 

Thank you.

Answer: 
When to Homebrew
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First, let's start with some theory. Ask yourself "why do I want to create this homebrew rule?" Is it because it provides for something the current rules don't, or because it sounds cool to make it? You should have a clear vision for what your rule provides that the existing rules don't. 

Current Options
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As far as you flavor goes, Pathfinder's core rules already contain exactly this idea. The [Sorceror](http://www.d20pfsrd.com/classes/core-classes/sorcerer/) is a class which explicitly gets its spellcasting power from its heritage. They are born with magical power lurking within them. Your proposed homebrew is redundant with existing options.

Another Issue: Scope
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Another consideration might be the difficulty of the proposed rule. Creating a homebrew item, spell, feat, etc. is easier because you haven't touched the underlying rules. In this case you are proposing a change to the fundamental aspects of a character. Any change here will have an influence on nearly every other aspect of a character. Attributes influence class abilities (including spells), skills, saving throws, feats, and more. Are you really ready to re-balance all of those based on your idea? 

To be clear you don't just need to re-balance the parts explicitly tied to spellcasting, you need to consider its effect on *everything*. If a Wizard uses your Magic attribute instead of Intelligence, that will also dramatically decrease their skill points. It will decrease their Intelligence-based skills. They may not qualify for some feats, which explicitly require a base Intelligence score. 

Alternatives
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All of my campaign settings take place in my own creations. I nearly always tinker with the default races, classes, etc. to fit the world I am creating. Generally, it is much easier to *cut* published material than it is to add to it. Just tell your players that for flavor reasons, the only full spell-casters available are Sorcerors.
The game system uses the six ability scores as its foundation. Tampering with this foundation by adding another ability score will create an enormous amount of work for you because the game assumes there are only six ability scores. This will have the following impacts.

1. **Every spellcaster (including martials) will become more MAD.** (*M*ultiple *A*bility *D*ependent.) Since this new ability score has no inherent value beyond spellcasting, this will result in all spellcasters needing more ability scores. This will be bad for gish classes and martials like the paladin because these classes already need physical ability scores to function.
2. **Creates a terrible dilemma for skill-based spellcasters.** Bards, inquisitors, and similar spellcasters that are primarily skill-focused will suffer because now there's an opportunity cost between investing in skill ability scores and investing in the Magic ability score.
3. **Less distinction between spellcasting classes.** Now that all spellcasters need the exact same ability score to function, you will see less distinction between spellcasting classes. No more can we assume sorcerers and bards will be more charismatic. Nor we can assume clerics will be wise or the wizards intelligent. This hurts the flavor of each class's method of spellcasting.
4. **Requires an enormous amount of new content changes.** You will now have to add new magic items, spells, feats, etc. that will interact with the new ability score. Races will have to be changed. This is a lot of new work for you that will ultimately not be very fruitful.

Overall, adding a Magic ability score will create an enormous amount of work for you and your players while not really accomplishing much aside from homogenizing the flavor of spellcasting classes and increasing the opportunity costs of being a spellcaster (of any kind) or a skill monkey.