D20:Subdual Substitution Feat

Any other metamagic feat, Knowledge (arcana) 5 ranks.

When employing a spell with the acid, cold, electricity, fire, or sonic designator, you can modify the spell to deal nonlethal damage instead of the indicated type of energy damage. The altered spell uses a spell slot of the spell?s normal level.
The altered spell works normally in all respects except the type of damage dealt.

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System Reference Document -> Feats