Harm's health reduction lasts forever?

I was reading the Harm spell, and I quote:
"You unleash virulent magic on a creature you can see within range. The target makes a Constitution saving throw. On a failed save, it takes 14d6 Necrotic damage, and its Hit Point maximum is reduced by an amount equal to the Necrotic damage it took. On a successful save, it takes half as much damage only. This spell can’t reduce a target’s Hit Point maximum below 1."

Now, is written nowhere that the maximum hit points reduction has a finite time, and the spell casting time is instantaneous, does this mean that potentially, a BBEG thet casts this spell the first time the low-level party fight it can force them to seek for either a "greater restoration" spell or stick with a 1hp character?

Edit: I just read the rules of long rest, and is stated that completing a long rest will restore your maximum HP if it was reduced

It will return to normal after a Long Rest. 

Regain All HP. You regain all lost Hit Points and all spent Hit Point Dice. If your Hit Point maximum was reduced, it returns to normal.

Greetings Nyal_the_fox,

Yes, you read the Harm spell correctly for what it does, and I see you already found the Long Rest section of the Players Handbook that addresses the loss of max HP.

So, it seems like you answered your own question, but to confirm, yes... If you were playing a low level character and were reduced to 1HP in that fight before you finished off the BBEG, you would only have 1HP the rest of that adventuring day, then once you rested for 8 hours, you would have all of your HP restored.

Cheers!

Quote from Nyal_the_fox >>

 

[...] Edit: I just read the rules of long rest, and is stated that completing a long rest will restore your maximum HP if it was reduced

 Based on previous threads in this subforum, I'd say it's the kind of thing that's easy to miss when reading the new PHB and MM :(

I just wanted to add, in case it's useful to anyone visiting this thread in the future, that some monsters have their own specific rules:

Quote from Tarod_ >>

Quote from Maruntoryx >>

In 2024, any hit point maximum reduction ends upon finishing a long rest, so stat blocks don't say anymore.

 Well, generally yes (e.g. Specter, Succubus, Wight), so you apply the Long Rest rules, but not always. Some monsters have their own specific rules. For example Blue Slaad, Mummy, or Otyugh.

Related threads:

- Ability Damage Rules (comment #10)
- Life Drain duration
- Monster Manual 2024: Missing text from Shadow? Is it connected to a rules change for short/long rests?