Hi, is my first post so I should start by congratulating to you FerretDev for making this very fun and addictive game alone, this said I should also point out that the maxHP damage mechanic I've just encountered is a kind of mechanic that I think can make many people run away from the game (me for sure)
The maxHP damage is not a problem in itself, the problem is the maxHP not going back to normal after a full rest, so after an encounter with 3 demons with such skill my whole party was ruined forever and that is just not fun, seeying the Nero with 12hp at full health basically means the game just became like if you have your party injured but there is no healing option in the game (cause he's stuck at 12/12).
I strongly suggest to remove this mechanic or make in such a way that max hp goes back to normal after rest.