A horde of devious, ferocious, tenacious, and atrocious creatures, beasts, and monstrosities to populate the northern realms of your fantasy. Get them while they're cold!
Real World: some Indians in the Amazon treat their eyes with a traditional potion applied with palm leaves. Brutally painful, the drug alters vision, giving the jungle's dense green walls greater texture and dimension. You could adapt this to desert or swamps, or other hard to navigate regions.