1 Kyros: Channelling enough mana to blow apart the mountains will take several hours.
2 Alvissa: And if you're interrupted?
3 Kyros: If I lose concentration for so much as a split second, all the mana gathered so far will be released in a huge explosion.
4 Alvissa: Oh.
4 Kyros: Sorry, did you say something?

Do not interrupt the wizard!

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2023-07-17 Rerun commentary: This is another way to play interrupting spellcasters in roleplaying games. They don't just lose the spell they were about to cast - it goes off in an unpredictable and hilariously dangerous way. If you want to add some spice to your D&D combats, you're welcome to adopt this rule!