Does the Whole Group Die when One Hero Dies?
You don’t lose when you lose 1 hero. You lose when your Flame reaches 0. If you are at Max Flame, you can even lose multiple fights (5 heroes deaths) before the run is lost. Since a complete wipe equals 30 Flame loss on a non boss encounter, and that you have a base Flame of 90. Note that, boss encounters cannot be skipped.
Flame as a resource and your “Run Health” would be close to irrelevant if it didn’t end the run once it hits 0. You could just spend all your Flame on Epic Enchants for instance, and never bother to heal, and as long as you would be winning the fights, you would be good.
There’s actually a Relic that allows you to do just that “Not lose when you Flame reaches 0 if you still have 1 hero alive”, but having this permanently in all runs would mean that you can just disregard how much Flame you have and not care about spending it what so ever as long as you win fights.
Also, in Endless since the format is different (you can’t heal your Flame at all), you only lose 30 Flame per wipe and no Flame per individual Hero Death.