One Step From Eden

Hell Modes are modifiers that unlock when you beat the game. They unlock in sequence, so you'll have to unlock and beat Hell Modes 1-13 before this. You can toggle multiple Hell Mode modifiers together but you only need the latest one turned on to unlock the next, so don't use multiple unless you really want the challenge.

Most of the modifiers are barely noticeable, so you can work on other achievements like beating the game with everyone, getting the different endings, and playing without collecting artifacts/cards along the way. However a few Hell Modes can be rough (7 was the worst for me) so try to figure out which character and strategy works best for you so you can fall back on them if needed.

Hell Mode 14 specifically sets your max HP to 1, which sounds extreme but you can override this with Shopkeeper who's starting artifact makes your max HP equal to your money. You still start at 1 HP, but the starting world should be easy enough for you to survive that you'll be able to heal before the boss. Once you're healed up it's just a basic Shopkeeper playthrough which you should be ready for if you've played enough to get to this point.