You can cast a spell at any level above its own.
Prerequisites: Heighten Spell, Spellcraft 20 ranks.
Benefit: As Heighten Spell, but there is no limit to the level to which the character can heighten the spell.
Normal: Without this feat, a spell can only be heightened to a maximum of 9th level.
Source: Standard Reference Document under Open Game License v1.0a, which updated this feat originally published in the Epic Level Handbook, page 58.