So after talking with one of my players we got on the topic of a magical meteor strike. After talking it out we initially decided that it would require, as a minimum, 9 challenge and 1 difficulty dice for the base roll. 5 purple for extreme distance, upgraded 5 times for blast, another 5 purple for a duration of a few minutes, and upgrade 4 for destructive/empowered. This is based on an on the fly meteor strike and not a build it slowly approach with rituals, implements, etc. after reviewing the core rule book on "Impossible tasks" I have come to the conclusion that the check would change to being formidable difficulty upgraded 5 times, using at least 2 story points to reduce the check and 1 story point to make the roll at all. I'm curious fellow GM's how you would rule on this.

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Unfortunately you didn't give us anything to work with. What's the goal of the meteor strike? You mentioned a ritual. is it combat magic or a ritual to destroy an enemy tower?

If it's combat magic, that's the attack spell with appropriate extras. If it's a ritual, then I'd say that's a utility spell.

But again, until we know what they want the meteor strike to do, there's not much we can do to help. Without a solid idea of what's asked for we'll all give our own opinions on what a "meteor strike" is.

apologies, it was a hypothetical build to destroy a city, as it is an on the fly spell it would be a combat spell, though I am curious how other GM's would set it up as a utility as well.

By the rules as written, the difficulty caps at formidable unless your gm employs the optional impossible rules. For that, all that changes from formidable is that you have to spend a story point.

Your GM can of course, modify it with enviRon mental factors and upgrades for being overly dangerous but that's at an individual's discretion.

What's plan B? If the roll fails. Do we care whether the city is blown up or not?

I would make it a spell requiring research in Game at various institutions if learning. The ultimate discovery would be that a macguffin is required that was last seen eons ago, more research and chasing down reclusive sorcerers follows, after a number of suspected locations are searched a final location is identified. A two week trek through the mountains ends in an ancient tower full of fun. At the top is the device which requires days of preparations.

A series of checks leads up to the final casting, the cumulative results determining just how catastrophic it is. Then as they watch the meteor shower in the distance a portal to another dimension opens in the tower, with the demonic form of the sorcerer who built the device emerging to congratulate them! A monologue ensues where the daemon reveals the true purpose m: H ell gates are opening throughout the city and a vast army of daemons are emerging... don’t screw with magic you don’t understand!

Aren’t you limited to spending 1 story point to reduce difficulty?

You may only spend a single story point on a Check, an impossiblta task uses that check for zero benefit. So you can’t upgrade the dice pool with one, you can’t activate any talents (before or after, if they influence the check) or special abilities, you just have your characteristic and skill rank plus any boost dice you can wrangle off the gm to succeed against 5 Difficulty dice.