Dungeons & Dragons 3.5 Feat: Aligned Theurgy

You can combine different sources of power when using spells that contain the same alignment descriptions (chaotic, evil, good, or lawful).

Choose one of your alignment-based cleric domains: Chaos, Evil, Good, or Law. You can cast spells with the related spell descriptor as if your caster level was the sum of all your spellcasting classes that grant spells with that alignment-based descriptor. The effect applies only to a single alignment descriptor chosen when the feat is taken. For example, a 3rd-level cleric/4th-level wizard with the Chaos domain and this feat can cast any spell with the chaotic descriptor as a 7th-level caster.
This does not affect your spells per day or spells known. It only increases your caster level when casting spells of the appropriate descriptor.

You can gain this feat multiple times. Its effects do not stack. Each time you take the feat, it applies to a new alignment-based domain and spell descriptor.