Altitude sickness

The high peaks do not welcome lowlanders; the thin air is the mountain's first refusal.

Dangerously thin air starts a couple thousand feet below the snowline and gets worse from there. Locals are accustomed to it, but even they must be careful. Endurance is tested just to keep moving or breathe; debilities set in, details get missed, and travelers can collapse entirely — lungs filling with fluid, brains swelling, rolling for Death's Door. Safe acclimation means ascending no more than about 1,000 feet per day; a tea steeped from purple drakewort can alleviate symptoms, but overdose can suffocate.

Kind Environmental or Procedural Threat