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).

Shadowcasting

Protection from the dark times of Earthfall was but one of Zon-Kuthon's “gifts” to the terrified people of Nidal. To ensure the grip of his Umbral Court would remain forever unbroken, the Midnight Lord bequeathed to his mortal servants the secrets of blending shadow and magic. Though most shadowcasters practice wizardry (and indeed, wizards can choose the shadowcaster archetype if they wish to fully embrace this style of magic), Zon-Kuthon's gifts can be used by any spellcasting tradition. Beyond the borders of Nidal, cabals practicing shadow magic haunt the fringes of Galt and Taldor, some in service of the Midnight Lord, and others serving only their own lust for power.