I'm having somewhat of a disagreement with my GM onto the nature of how Frost Fall works, and when the secondary damage occurs. At level 4, I'm under the impression that the area of cold damage that triggers should last two rounds- That is, I cast it, at the start of my next two turns, it does the secondary damage twice before vanishing. My GM believes it should only occur once. It says at the start of my turn- That's also normally when I was under the impression spells with a duration of rounds end. But does the spell's effect happens first, then vanishes, or does the spell end without the secondary damage occuring?
it has a duration of 2 rounds at level 4, so it works for the round in which you cast it (from the moment you complete your standard action to cast it until it is your turn again) and THEN a second round, until it is your turn once again, at which point it ends directly BEFORE your turn.
So from my reading, it only damages at the beginning of the turn following the casting, bevause at the next trigger point "start of your turn" 2 rounds after casting, it is not in effect any more.