Rise of the Rune Lords
Substitute Spell Power
You may sacrifice one spell in order to increase the difficulty to resist another that you are casting.
Prerequisites: 12 or more ranks in Spellcraft, mystic theurge level 5th
Benefit: When you cast a spell, you may sacrifice a higher level uncast spell or unused spell per day (for spontaneous casters) in order to increase the Difficulty Class of the saving throws to resist spell that you are casting. The spell’s Difficulty Class increases to what would have been the DC of the spell that you sacrificed, even if it was calculated using a different ability score (i.e., if you sacrifice an arcane spell to make a divine spell more powerful). Alternatively, you may sacrifice a lower level spell as you are casting a spell in order to increase the Difficulty Class of the spell by 1. You gain no benefit from sacrificing an orison or cantrip.
From Undefeatable 18 – Mystic Theurge, page 4.