Greater Spell Focus

Greater Spell Focus [General]

Choose a school of magic to which you already have applied the Spell Focus feat. Your spells of that school are now even more potent than before.

Benefit: Add +1 to the difficulty class for all saving throws against spells from the school of magic you select. This bonus stacks with the bonus from Spell Focus.

Special: You can gain this feat multiple times. Its effects do not stack. Each time you take the feat, it applies to a new school of magic to which you already have applied the Spell Focus feat.

Source: Player's handbook