Uncanny Forethought( Exemplars of Evil, p. 26)
You cunningly prepare your spells for any exigency.
Spell Mastery (PH) , INT 17,
When preparing your daily allotment of spells, you can reserve a number of spell slots equal to your Intelligence modifier. As a standard action, you can use one of these slots to cast a spell that you selected for the Spell Mastery feat. The level of the slot used must be equal to or greater than the level of the spell you intend to cast.
Alternatively, as a full-round action, you can use a reserved slot to cast any spell that you know. The spell is resolved as normal, but for the purpose of the spell, your caster level is reduced by two. The level of the slot used must be equal to or greater than the level of the spell you intend to cast.