whats the damage record for a single spell? I just did the math on this and it does 4193280000 cold and 4193280000 negative energy and 10080000 negative levels over the course of 100 weeks, so it's kinda slow but I cant think of anything that is not immune(ex skeleton) that can survive being trapped in this for its duration
Unfortunately, we figured out a couple of pages ago that Fell Drain and Flash Frost don't repeat- unless the underlying spell does repeating damage itself. Since Fimbulwinter doesn't do ANY damage by itself, they would happen once and that's it. The only way I could find to do repeating damage, as part of the spell effect, was using Bracers of the Entangling Blast- which only last 1d3 rounds- for a max of 88 damage and 4 neg levels with all the fx. Unless the Entangling Blast is Maximised and Empowered like the rest of the spell- which, btw, should Empower the Flash Frost as well, so all the damage should be bumped up by 50%. In that case the grand total would be 132 damage (half cold/half neg) and 4 neg levels. Enough to kill even 8th level barbarians- not to mention all the peasants, livestock and wild animals that probably dropped in the first round.
PS- One should note that the blizzard that Fimbulwinter creates should definitely be in the "Extreme Cold" category so it will do 1d6 lethal damage every minute. That's 1,051,200 - 6,307,200 over the course of 2 years. But that's not from the spell (directly) and I doubt it's the record anyway.
PSS- On a sadder note, one must also consider that wights are not immune (or even resistant) to cold, and so are unlikely to escape the area of effect after they rise and wind up frozen to death just like everybody else. SO the overall effect is more of a Nuclear Fimbulwinter after all, instead of a Wight Christmas.