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I've noticed that the coastal mainland can support crops but the mainland interior is just too cold for that. Most barbarians live in the interior. The corebook notes that barbarians tend to raid Khromarium during the winter. My conclusion is that they're after the food stores. It's also a good way of thinning out the weak among the barbarians without resorting to a formal cull.
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There have been mentions of Underborea and the possibilities of alternate crops (Mushrooms, Other Fungus, Alge, Livestock). But Raiding for Stores would be probably a steady reason to come down from the mountains.
Bob
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Winter is the most perilous NPC on Hyperborea. It most probably is the biggest killer in the setting.