Pathfinder - The Mystic Theurge Handbook
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I will use the color coding scheme which has become common among Pathfinder build handbooks. Also note that many colored items are also links to the Paizo SRD.
- Red: Bad, useless options, or options which are extremely situational.
- Orange: OK options, or useful options that only apply in rare circumstances
- Green: Good options.
- Blue: Fantastic options, often essential to the function of your character.
The Mystic Theurge is a popular concept which followed Pathfinder out of 3rd edition. The class saw significant improvements in the transition which make the class much more playable, but the Mystic Theurge is by no means simple.
Mystic Theurges handily address one major problem with spellcasters: the limited number of spell slots. You will have enough spell slots that you can comfortably cast spells every combat round all day long and will only rarely run out. However, this comes with its own problem: By splitting your focus you'll fall behind on both caster level and spell DC. Because of this, Mystic Theurges need to rely on spells which don't allow saving throws to make their huge pool of spell slots meaningful.
Mystic Theurge Class Features
Hit Points: d6 hit points means that you're very squishy. Most of the arcane spellcasting classes you'll use can't cast spells in armor, which further compounds your defensive issues.
Base Attack Bonus: 1/2 BAB, but don't expect to use a weapon at any point.
Saves: Only strong Will saves, but fortunately you're working from two base classes so you'll have strong save bonuses before you enter the prestige class.
Skills: 2+ skill points and a tiny skill list which you'll almost certainly already have as class skills.
Weapon and Armor Proficiency: You're not here for armor or weapons.
Spells per Day: This is why you play a Mystic Theurge. Remember that classes like Witch and Wizard will need to find other ways to learn new spells, typically by copying them from other spellcasters' familiars/spellbooks.
Combined Spells (Su): This is perhaps the greatest incentive to combine a prepared caster and a spontaneous caster class. If you use only prepared caster classes you can still prepare spells from one class using the other class's spell slots, but you gain very little by doing so.
Spell Synthesis (Su): Even with handicapped spell levels, this is more powerful than Quicken Spell and you can do it without raising the effective level of your spells.
You need two base classes: an arcane class, and a divine class. Pathfinder's list of base classes is dizzyingly long, so you have a lot of options.
Combined Spells has some interesting interactions for combining spontaneous and prepared spellcasting, but beyond that the only significant difference is how quickly you can get into Mystic Theurge levels. Personally I prefer to stick to prepared classes because you can start Mystic Theurge at level 7, but there are some excellent reasons to wait until 8th or 9th level.
While you can qualify for Mystic Theurge using partial spellcasters like Bard and Magus, I do not recommend doing so. You'll already be several levels behind full casters in terms of your highest spell level, and using partial caster classes further compounds the issue. These classes typically get other class features instead of spellcasting, and your Mystic Theurge levels don't advance those features. I'll ignore all classes except 2/3 classes for these reasons.
- Arcanist: Intelligence-based with a unique preparation/casting mechanic. The Arcanist's unique spellcasting has a weird interaction with the Mystic Theurge's Combined Spells feature. As far as I can tell, you prepare Arcanist spells, and if your divine class is spontaneous you can use those spell slots to cast any of your prepared Arcanist spells. Aside from this amusing interaction, the Arcanist is basically just a worse version of Wizard with the Exploiter Wizard archetype. You get one more Arcanist Exploit than a Wizard could, but you get spells a level later and you enter Mystic Theurge a level later. If you want more exploits, take Extra Arcanist Exploit. Because Arcanists need Charisma for their exploits, consider pairing the Arcanist with the Oracle.
- Sorcerer: Charisma-based spontaneous. You get your first and second bloodline powers before you switch into Mystic Theurge.
- Bloodline: Celestial (Wildblooded: Empyreal): This options switches the Sorcerer's spellcasting ability to Wisdom. Every divine spellcasting class is Wisdom-based except the Oracle. Suddenly all of your spellcasting is Wisdom-based, which makes it much easier to boost your spell DCs, bonus spells slots, etc.
- Witch: Intelligence-based prepared. You get two hexes and a familiar. Your familiar is going to spend a lot of time hiding because its abilities won't advance, and many Witch hexes depend on Witch class levels to remain effective. However, there are some notable exceptions like Evil Eye, Fortune, and Misfortune which remain useful.
- Wizard: Intelligence-based prepared. Wizard spellcasting is among the best in the game, but you won't get much out of other class features. Arcane Bond's item option will work great, but avoid familiars and pick a school with a good 1st-level ability.
- Archetype: Exploiter Wizard: Arcane Pool can be a helpful way to boost your problematic caster level and spell DCs. You only get one Arcanist Exploit, but a few are still worthwhile. Potent Magic will double the effectiveness of using your Arcane Pool to boost your caster level and spell DCs, which is nice since you won't get more than 4 points.
- Archetype: Spell Sage: The ability to boost your caster level will work better as an Exploiter Wizard, and Spell Study is pointless since you're already going to be taking a divine spellcasting class.
- Cleric: Wisdom-based prepared. Your cleric class features aside from spellcasting will do very little for you, but the spell list is the real draw. It's interesting to note that because your domain spells are prepared, you can use spell slots from your arcane casting class to cast those spells more than you might be able to if you were a single-class cleric.
- Archetype: Ecclesitheurge: You won't be able to wear armor or shields or use weapons anyway, so giving them up costs you nothing. You don't get a lot back that the Mystic Theurge can't already do, unfortunately.
- Druid: Wisdom-based prepared. Druids have a really neat spell list with a lot of really interesting options, but their low-level class features aren't helpful and they're really dependent on having high-level spells to make their spellcasting effective. I would take a Cleric with the Animal domain long before considering druid.
- Oracle: Charisma-based spontaneous. The primary reason to choose Oracle over Cleric is for Revelations. Some of them are fantastic without needing to scale based on Oracle class levels. However, you're still going to be a spell level behind cleric, which may be a deal-breaker for you. If you want to rely on a single ability score, Oracle-Sorcerer is an option.
- Dual-Cursed Oracle: The Mistfortune revelation is insanely good, and it could help compensate for your relatively poor save DCs.
- Shaman: Wisdom-based prepared. The Shaman is a weird class. Their spell list looks like the Druid's, and they get hexes like the Witch. The spell list is still the primary draw here, so the Shaman has all the same problems that the Druid does.
Your ability scores are going to vary depending on your base classes. Generally you want to start with 16 or better in each of your spellcasting abilities, and put whatever you have left into Dexterity and Constitution.
Your choice of race depends entirely on your base casting classes. If you can find a race which works well for both of your base classes, you've found a great race. If you find a race that works for at least one of your base classes, you've found a good race.
Mystic Theurge is among the only builds where I recommend the Human variant racial trait which gives you two flexible +2 ability score increases. There are very few races with +2 to two mental ability scores, which is what you want most from a race.
- Magical Knack (Magic): You're going to give up 3 or 4 caster levels from two classes, and this is one of very few ways to recover the missing caster levels.
Your skill options vary too much for me to meaningfully address. Your list of skills and your ranks will vary wildly depending on your choice of base classes. Skills are secondary to the class's function, and I fully believe that you are capable of sorting out your skill ranks based on what you know of your base classes.
- Spell Penetration: Late in the game a lot of monsters will have Spell Resistance. You will need this feat at some point.
- Greater Spell Penetration: Spell Penetration is likely plenty, but another +2 never hurts if you have a free feat.
Stop. Put that down. You need your hands for spellcasting.
Armor is presented in the order in which you should acquire it, rather than alphabetical order.
- Haramaki: Cheap, enhanceable, and no Arcane Spell Failure chance.
- Mage Armor: Hours/level duration and a decent AC bonus.