From publisher blurb:
Mystic theurges use arcane methods to explore their spirituality and faith, and then seek within this faith for secrets and abilities that can be applied to their arcane studies. This blurring of the traditional lines of esoteric and faith grants mystic theurges incredible powers of arcane and divine spellcraft. They have chosen a synergized approach to magical power, more diverse and potentially more potent than the traditional paths of clerics and wizards.
The mystic theurge is probably the second most debated change to the 3.5 rules set (right after the changes to weapon sizes). Some feel the class is too powerful, others too weak - regardless, the class bends and blurs the lines between arcane and divine spellcasting.
Within this short product, the Mystic Theurge is expanded upon, three additional paths of theurgy are explored (elemetal theurges, the lord of undeath, and the theurgic lorewarden), as well as the quintessential master of theurgy - the heirophant theurge. The product is rounded off with a small selection of theurgic feats and magic items appropriate to a game where theurgy is is a central element, either through player involvement in the class or through adventures about theurgists and their churches.
For those who find theurgy overpowered, there are reduced power variants and rules for reducing the power of the 'core' mystic theurge also to be found within this product.