We're playing D&D 3.5 edition. Some undead, like vampires and wights, are level drainers. 3.5 rules have a simplified set of penalties for a "Negative Level" imposed on the victim for the first 24 hours. After that the victim gets a saving throw to see if they slough off the penalties, or-- if they fail the save-- suffer an actual level loss. You can read rules online at https://www.d20srd.org/srd/specialAbilities.htm#energyDrainAndNegativeLevels. Either way, too, there are spells that can reverse the loss.

Wraiths are a bit different in that they are drain points from the Constitution ability. Their attack allows an immediate save to avoid the drain, but a victim who fails that save loses Con points immediately. Details for the rule are at https://www.d20srd.org/srd/specialAbilities.htm#abilityDrain. And again, there are spells that can reverse the loss. Terence happened to have one on a scroll (it's 1 level higher than he can cast) as a "Break Glass In Case Of Emergency" measure.