You blend shadow into your spells, increasing their efficacy at the price of susceptibility to light.
Benefit: When you cast a tenebrous spell in darkness or dim light, the spell’s effective caster level and any associated save DCs are increased by 1. Any attempts at dispelling a tenebrous spell in darkness or dim light take a –2 penalty on the dispel check.
Casting a tenebrous spell in bright light is difficult, and requires a concentration check (DC 15 + twice the tenebrous spell’s effective spell level). Attempts to dispel a tenebrous spell in bright light gain a +4 bonus on the dispel check.
You cannot use this feat on spells with the light descriptor. A tenebrous spell uses up a spell Slot one level higher than the spell’s actual level, except in the case of spells with the darkness or shadow descriptor or of the illusion (shadow) subschool; this feat does not change the effective level of those spells (but still counts as using a metamagic feat for all other purposes).