Born of the Three Thunders( Complete Arcane, p. 76)
You have learned to marry the power of lightning and thunder in your electricity and sonic spells.
When you cast a spell with either the electricity descriptor or the sonic descriptor that deals hit point damage, you can declare that spell to be a spell of the three thunders, with half its damage dealt as electricity damage and half dealt as sonic damage. In addition, the spell concludes with a mighty thunderclap that stuns all creatures that take damage from the spell for 1 round unless they succeed on a Fortitude save, then knocks stunned creatures prone unless they succeed on a Reflex save (both saves at the same DC as the base spell). Channeling the three thunders is costly, though, and you are automatically dazed for 1 round after doing so. A three thunders spell uses a spell slot of the spell's normal level. In addition, its descriptor changes to include both energy types--for example, a lightning bolt of the three thunders is an evocation [electricity, sonic] spell.