Greater Spell Focus( Epic Level Handbook, p. 69)
Choose a school of magic, such as illusion. Your spells of that school are far more potent than normal.
Spell Focus in the school selected,
Add +4 to the Difficulty Class for all saving throws against spells from the school of magic you select to focus on. This benefit overlaps (does not stack with) the bonus from Spell Focus.
You can gain this feat multiple times. Its effects do not stack. Each time you take the feat, it applies to a different school of magic.