Extra Slot( Ghostwalk, p. 32)
You can cast an extra spell.
Spellcaster Level 4th,
You gain one extra spell slot in your daily allotment. This extra slot is one level lower than the highest-level spell you can cast. For example, a 6th-level sorcerer gains an extra 2nd-level slot. Once selected, the extra spell slot never changes level. For example, once the 6th-level sorcerer above becomes a 10th-level sorcerer, this feat still gives him an extra 2nd-level slot (because this feat was taken at 6th level), not a 4th-level slot (one lower than his current highest-level spell).
If you have more than one spellcasting class, the extra slot only applies to one of those classes.
You can gain this feat multiple times. Each time, you gain an extra spell slot at one level lower than the highest-level spell you can cast.
This feat first appeared in Tome and Blood