[RPG] What are the implications of allowing Dex to Hit and Damage

No, for three reasons:

First, Rapier specifically states:

You can’t wield a rapier in two hands in order to apply 1½ times your Strength bonus to damage.

Second, Weapon Finesse allows you to use DEX mod instead of STR mod on attack rolls. Wielding a one handed weapon in two hands allows you to add to your damage rolls. Two completely different cases.

Third, you cannot use Weapon Finesse when wielding two handed weapons, or wielding a one handed weapon in two hands. There are a couple of specific items that change this ( iirc, Elven Courtblade from Races of the Wild and the cheesechain Spiked Chain ), but it is limited to those items.

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When optimizing a character, it is key to identify what you will be spending most of your time doing.

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In this case, it seems like you want to make a melee character who fights in melee. The problem is, as a Magus, you will be combining spell use with melee attacks, and relying on spells to do your damage or to attach Saving Throw based effects. However, your melee attacks still need to hit to apply these spells. So we need to optimize two things - your to hit and your spell effects.

Why not rely more on melee damage? As a non-Fighter and non-Barbarian and non-Rogue, your melee damage is low. You can't easily use PA to increase your damage as your to-hit is key. Ergo, relying on pure melee damage is a bad idea with this build.

To-Hit

To Hit is made of three parts -

• Stat added to to-hit
• spell bonuses and gear bonuses to hit
• feat bonuses to hit

Spell and gear bonuses are simple - you select spells (such as Haste and Cat's Grace) that give you bonuses to hit and try to have them cast before combat goes up. Additionally, you buy or steal or barter or trade or select items (gloves of dexterity, magical swords, banners, duelist's gauntlets, sashes etc) that increase your to-hit. It's usually fairly binary, and choices between say, a great save or die to apply via sword hits or a spell that increases the chance of hitting, can be decided upon by comparing the great spell to your pre-existing spells, thinking about how often you hit currently, etc.

Stat added to hit is also fairly simple. You want more of the stat that adds itself to your to-hit, within reason, again, by comparing it to other things you might get with the same money.

Feat bonuses are slightly more complex, as some feat chains offer debuffs which can affect to-hit, and computing the result is non-trivial. However, as you aren't a Fighter, you can't afford enough feats to make the Improved Trip line worthwhile (Expertise, Trip 1, Trip 2, Fury's Fall), as you aren't a rogue the Shatter Defenses line isn't worth it, and most of the others aren't great shakes. Weapon Focus is an okay choice as it pays off immediately, doesn't require anything else, and is easy to use with your weapon choice of 'always scimitars'. The Two-Weapon Fighting chain is mandatory. That leaves you with very few feats, so the only real 'bang for buck' feat choice is Dazing Assault. Daze is a great status effect and worth applying.

Spell Effects

Here we start to run into problems. In the proposed build, you have 4 lost caster levels. In a partial casting class like the Magus, that's effectively suicide. Unless there are some vastly powerful low level spells in the Magus list that you are going to be able to rely on, and you're taking the levels in those classes later - which there is no sign that you are, as they appear to be low level dex and BAB boosting classes.

Unless you are going to dual-wield scimitars and take power attack/twf chain etc, I recommend you only take one level instead of four. Taking a second level later might be workable, but 4 levels of delayed casting as a Magus will suck.

If you have 3.5e backwards compatibility, things like the Abjurant Champion prestige class might help you out in getting a better BAB while maintaining your casting progression (although you still lose out on arcane pool and arcana - the only worthwhile things about advancing pure magus).

Now, as for advancing your spell effects, we have two main things to worry about - save DC and damage.

Save DC is increased by int and various feats, none of them amazingly good. Spell Specialization is a good choice, as is spell Focus in a school you have multiple good options in (like Conjuration or Transmutation).

Damage is increased by CL and metamagic feats. The Gifted Adept and Metamagic Master traits are key to increasing the power of a spell you'll use a lot, such as Shocking Grasp. Otherwise Spell Focus, Mage's Tattoo (Varisian Tattoo), Spell Specialization, can all increase your CL. At lower levels, an Empowered Shocking Grasp for 5d6 x 1.5 damage is crazy at level 4 or whatever. At higher levels, an Intensified Empowered Shocking Grasp out of a 2nd level spell slot for 10d6 x 1.5 damage is great in addition to a full attack.

Taking a level in Crossblooded Sorcerer (Draconic/Orc, or anything else that boosts damage) can greatly increase the damage of touch spells you use also.

Once you have all that handled, damage, status effects, how you are applying them, then you can think about ratios of int to dex.

And in this case, mathematically speaking, you want 2 more points of Int than Dex at any one time, but both are important.