You have certain spells which, when cast using an alternative spellcasting ability, are more powerful.
Benefit: When you take this feat, choose a number of spells that you can cast equal to your mystic theurge level. Whenever you cast one of these spells using your combined spells class feature to cast it with one of your other spellcasting classes, the spell is considered to be heightened (as the Heightened Spell metamagic feat) to the level of spell slot that was used to cast it. Alternatively, you may choose to cast or prepare these spells using the same level of spell slot from another spellcasting class, rather than a spell slot 1 level higher than the spell. Whenever you gain a level in mystic theurge, you may add 1 more spell to this group. (The number of spells you can cast in this way will always be equal to your mystic theurge level.)
Normal: Normally when you use the combined spells ability, you must use a spell slot 1 level higher than the spell you are casting.
Undefeatable: The Collected Feats Sourcebook, Copyright 2009 – 2010, Louis Porter Jr. Design, Inc. Undefeated, Copyright 2011, Louis Porter Jr. Design, Inc.