Merciful Spell

Metamagic

Your damaging spells subdue rather than kill.

Benefit: You can alter spells that inf lict damage to inf lict nonlethal damage instead. Spells that inf lict damage of a particular type (such as fire) inf lict nonlethal damage of that same type. A merciful spell does not use up a higher-level spell slot than the spell’s actual level.

Source: Advanced Player's Guide