Heroist
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Gentlemen, replenish your mana and cast Dispel Magic. It seems you need to do this before entering the fog. If there's a visible boundary, cast it there. The fog drains mana, and who knows what else it does; it's better not to go in unprepared.
[I drink a potion, head north, cast Dispel Magic, head north, and still one-shot the fireflies.]
Added 2 minutes later
a warlock capable of dispelling magic could destroy this obstacle, but...
Whatever