Mystic Theurge

The Mystic Theurge is a Prestige Class from Dungeons & Dragons 3rd edition and the Pathfinder corebook. It can be summarized as, "I want ALL the motherfucking magic!"

Seriously, that's all there is to it. A Mystic Theurge is a PRC that improves your capabilities as both a Divine Spellcaster and as an Arcane Spellcaster. Great if you really can't choose between being a Cleric and a Wizard, or an Oracle and a Sorcerer, or whatever.

To get into this Prestige Class, you need to be able to cast at least 2nd level Arcane spells AND 2nd level Divine spells. You also need either 6 (for D&D) or 3 (for Pathfinder) ranks in BOTH Knowledge (Arcana) and Knowledge (Religion). In return, you get a ten level PrC where each level gives you spellcasting improvement in both your Arcane spellcasting class and your Divine spellcasting class.

3.5e

Mystic Theurge is considered to be, when played as presented, not a very powerful class. Using just the core rulebooks, the fastest way to get 2nd-level arcane spells is with 3 levels of wizard and the fastest way to get 2nd-level divine spells is with 3 levels of cleric or druid, so your first level of mystic theurge begins at character level seven. If you had simply taken seven levels of wizard instead, you'd already be casting 4th-level arcane spells; if you had seven levels of cleric or druid, you'd be casting 4th-level divine spells. Generally, this is considered a much bigger deal than being able to spam lots and lots of 2nd-level spells from both spell lists. Then, from that point onward, each level of Mystic Theurge ONLY pumps up your BAB, saving throws, and spells per day. It does not progress the class features of whatever base classes you build it on, nor does it have any class features of its own, so you'd better build on on top of classes that get all their class features at level 1. At this point, it becomes critically important to remember that access to high-level spells is never gated directly by class level. It's always done indirectly, and it's different for every class. Most prepared casters, like the Cleric and Druid, do not bother with "knowing" spells, and are gated only by their Spells Per Day charts; they benefit fully from Mystic Theurge (other than the Druid being locked out of its higher-level class features). Most spontaneous casters, like the Bard and Sorcerer, are gated by both a "spells known" and "spells per day" chart. Most prestige classes that advance an underlying caster level will advance both spells known and spells per day, but the Mystic Theurge only mentions spells per day, so these classes are screwed. The Wizard is gated by its spellbook, which in turn is gated by its spells per day chart; in theory, a character could add two new 9th-level spells to their spellbook upon reaching their second level of Wizard, if their "spells per day" chart had been sufficiently inflated by prestige classes between taking their fist and second levels of wizard. However, Wizards do not learn new spells when taking a level in a prestige class, even if that prestige class boosts spells known. The Warmage, Dread Necromancer, and Beguiler know their entire spell lists, so their access high-level spells is limited only by their Spells Per Day chart; they fully benefit from Mystic Theurge, aside from being locked out of their higher-level class features. Then there's the Spirit Shaman, which has two "Spells per day" charts of different kinds, so fuck trying to figure out the ramifications of that.

The list of arcane and divine base caster classes that get all of their class features at level 1 is actually pretty short. There's the Cleric and Sorcerer, and what else?

What is interesting to note is that neither Mystic Theurge nor the game itself defines just what arcane spellcasting classes and divine spellcasting classes are. With all the mess later in the game this line was rather muddled, putting all those fancy variety spellcasters in a sort of limbo regarding just what they are. This means that if a class is somehow both an arcane and a divine caster at the same time, then each level of mystic theurge can add both of its "+1 to arcane/divine spellcasting class" boni to the same class, effectively doubling how fast they level up their casting.

One way to do this is by ways of the Rainbow Servant. After going through the full 10 levels of this Prestige Class, an Arcane caster is capable of learning and casting Cleric spells the same way they cast their regular spells (note: they gain access to Domain spells earlier than this, but Complete Divine page 20 specifies "such spells are considered arcane spells when cast by arcane spellcasters." Only the Cleric spells granted at Rainbow Servant level 10 are cast as divine spells). Regardless of how well it works for the various classes with access to the class, this means that your base class is now both an Arcane and a Divine caster at the same time, and those ten levels of Rainbow Servant have also advanced the underlying arcane caster's spells per day and spells known by 6 levels. If you prestiged into Rainbow Servant by way of a level 3 Wizard, for example, then you're a level 13 character with the spells per day of a level 9 Wizard. From this point onward, just start stacking Mystic Theurge, choosing your base arcane caster class (not Rainbow Servant!) as both your arcane and your divine caster class to progress. It will take four levels of MT to "catch up" to where you should be. After that, you start seeing profits. Using a level 3 wizard as an example again, the first 4 levels of MT will finally get you a level 17 character with the spells per day of a level 17 wizard; after that, you're a level 18 character with the spells per day of a level 19 wizard, then a level 19 character with the spells per day of a level 21 wizard, then a level 20 character with the spells per day of a level 23 wizard.

If your DM is drunk or just not paying attention, then there's another fun way to break MT using Rainbow Servant. Take the requisite 3 levels of wizard and 3 levels of druid, or 4 levels of Bard and 4 levels of Dread Necromancer, or whatever arcane and divine caster classes float your boat. Then take ONE level of Mystic Theurge, and then start stacking Rainbow Servant, choosing Mystic Theurge as the divine casting class to progress. If you're lucky, your DM will allow this bullshit to fly, allowing MT to convert ten levels of Rainbow Servant into 6 levels of both your arcane and divine caster classes while still gaining rainbow wings, three Cleric domains, and access to the entire Cleric spell list. However, if your DM is autistic or otherwise has a firm understanding of the rules, he or she will correctly point out that MT has no spells known or spells per day of its own, and is therefore not a caster of any type, and therefore cannot be targeted for progression by Rainbow Servant. They may also point out that even if MT did have some spellcasting ability of its own, its boosting of the underlying caster classes is a class feature, and class features can only be progressed through taking real levels in MT.

Pathfinder

The Pathfinder version is a little meatier, in that it gains access to the Combined Spells class feature (which basically lets you give up one class's spellslot to prepare a spell from the other class of one level lower) and the Spell Synthesis class feature (which lets you cast an Arcane spell and a Divine spell simultaneously once per day).

5e

In 5th edition, with the general approach of "class archetypes over prestige classes", the Mystic Theurge currently exists in four different options:

• The Arcana Cleric Domain provides Clerics with a selection of bonus Wizard spells and some "wizardy" Domain bonus features.
• The Favored Soul returned as a Sorcerous Origin called the "Divine Soul", which grants sorcerers some "angelic" traits and the ability to cast cleric spells.
• The Celestial Pact Warlock has some bonus Cleric spells focused on healing and can even heal with a touch.
• The Theurgy Tradition for Wizards grants the slow access to Cleric spells and the bonus class features of a chosen Cleric Domain. This is currently an "unofficial" option, as it exists only in Unearthed Arcana.