Full text of ability:
Feat of Constitution (Su): You can expend one use of
mythic power to attempt a feat of Constitution, gaining a
+20 circumstance bonus on any one Constitution ability
check. Alternatively, you can use this ability to gain a
+20 circumstance bonus to your Constitution score for a
number of hours equal to your mythic tier, but only for the
purpose of making Constitution checks against heat, cold,
fatigue, and exhaustion.
As far as I can find there are no Constitution checks against heat or cold, they require Fortitude saves instead. Only fatigue, exhaustion, suffocation, thirst, and starvation seem to require Constitution checks. I imagine the intent was to give a +20 on those Fort saves. Anyway, just a minor issue I wanted to make note of.
