In the Road to Legend, the plot "Ascension" there is the "The Harbinger Approaches" card. It states that if the four temples are in play, you place comets on temples with the rule:

All heroes have their maximum wounds and fatigue reduced by one for each comet token on a temple within three trails of their current location.

What happens if an 8 max-health hero is within three trails of a temple that has 8 comet tokens placed on it?

I wondered that myself, but none of the heroes in our game have only 8 health so didn't look too closely. Presumably they die the instant they get within reach, though I'm guessing FFG would have some sort of ruling that prevents instant infinite XP for the overlord if the comets are within 3 of Tamalir. Perhaps it only matters when in a dungeon or encounter. Or perhaps they'll add a minimum of 1.

Long story short: tell the 8 health hero they'd better buy a +4 health boost before that happens.