I'd like to take Born of the Three Thunders when my sorcerer hits level 6.

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.
My DM has ruled, though, that if I cast a damage-dealing spell using BotTT that has a duration longer than instant, that the thunderclap / stun / knockdown effect occurs when the duration ends; that is, he rules that "the spell concludes" means "when the effects of the spell end", not "when you finish casting". So if I drop, e.g., Arctic Haze with a duration of 1 hour per level on them, the stunning thunderclap occurs 6 hours in the future - but still works, even if they are now miles away.

Is this consistent with how others see the feat? Is there a WotC ruling somewhere?

Secondly, per DM, the dazed penalty hits IMMEDIATELY even if the spell has a duration. My question is, when the feat says "you are automatically dazed for one round", does that make you dazed until the end of this round, until the start of your next turn, or until the end of your next turn? I'd assume the latter, but again, a WotC ruling would be helpful.

Let us all now engage in a 3 Minute Hate of just how poorly WotC writes. This is far from the most confusing feat anyone is going to come across, but I'm hating on it, because it fits my character so well and yet is crippled by poor writing.

BTW, if anyone can come up with spells for me that
  1. Do Acid / Cold / Fire / Electric / Sonic damage (Energy Substitution [Electric] a pre-req for BotTT)
  2. Have an area effect so I can hit multiple targets
  3. Either have no save or a save that isn't Reflex (because Evasion is a thing)
  4. Are instant duration



I shall be most grateful, and your karmic cup shall overflow with goodness.

Yes, I know of Mettle. It's not as common as Evasion.