So i reduced a players health by almost half today, im curious how that resets or if it does not reset after the combat?

this was reduced by the vampire spawn ability in curse of strahd

As Bite say, ''The reduction lasts until the target finishes a long rest.''

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

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?

Sorry i didn't know which version but as Maruntoryx said the 2024 the rules for Long Rest state;

 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.