Crucible of Souls
This does NOT work like you may expect it to from other rules and it’s likely to be different for every monster. It’s still dangerous, even lethal, but now has nothing to do with a characters actual experience points, and likely to be just as dangerous to characters of ANY level (less dangerous to lower level characters, possibly more to higher level characters).
Usually, if an undead drains or damages something below half its normal maximum it means the character is cursed. This drain will not in and of itself necessarily kill the character, but unless something is then done to remove the curse, when the character actually dies thereafter, they could rise to become undead of the appropriate type. Obviously, if you ARE killed by an energy drain of some kind you’ll turn into an undead. Energy drains might reduce different ability scores, hit points, or have some other kind of effect entirely.
The ability point losses are very slow to recover. Complete bed-rest is pretty much required – no combat, spellcasting, or heavy exertion (including wilderness travel). With sufficient rest characters recover 1 ability score point per week.
Recovery of ability scores will not affect an actual curse! It only restores the ability score. Characters will feel lingering effects from curses but lifting any kind of curse is left to players to discover how it is to be accomplished. Actually having a Remove Curse spell for the character or the use of holy water is obviously a good start, but each type of undead and its unique attack may have a somewhat different process or additional requirements for lifting its curse.