Café de Nuit

For curiosity's sake, the child vampire (AD&D 2e Darklords, page 60+) above;

90 years old and looks like 10. Also keeps company with Adam, Lamordia’s lord. She is not a darklord or a prisoner of of domain.

Human Vampire, Neutral Evil,
Armor Class 1
Movement 6
Level/HD 4+3
Hit Points 18
THACO 15
No. of Attacks 1
Str 5 Con 5 Dex 15 Int 19 Wis 19 Cha 17
Damage/Attack: 1d3
Special Attacks: See text
Special Defenses: See text
Magic Resistance: Nil

As a human vampire, Merilee has many of the normal powers and abilities associated with such creatures. Because of her child's body, she does not possess the magnificent physical strength (and associated melee bonuses) normally found among the undead. Merilee’s gaze is, conversely, more powerful than that of a normal vampire. This is, it seems, due to the fact that her fictional innocence conveys a sense of helplessness and sympathy to her victims. Thus, all who roll saving throws against her charm power do so with a —4 penalty. While Merilee can transform herself into a wolf or bat, as can normal vampires, the animal forms she assumes are as immature as her natural one. Thus, she appears as a wolf pup or a young bat. The creation of other vampires is forever beyond the power of Merilee. The infusion of Negative Material plane energy required to accomplish this is draining for even a normally robust vampire and would certainly be fatal to so fragile a specimen. Merilee’s high Intelligence and Wisdom scores bestow upon her certain special immunities above and beyond those normally associated with a vampire. She is, for example, wholly immune to the effects of 1st-level illusions. Further, spells such as cause fear, command, friends, and hypnosis have no effect upon her. Of course, as a vampire she is already immune to all manner of charm, sleep, and hold spells. Apart from the differences above, Merilee's other vampiric powers function just as they do for normal undead creatures.

Combat Tactics: Merilee, keenly aware of her own physical weaknesses, seldom engages in toe-to-toe fighting. Instead, she uses her cunning and innocent appearance to lure victims close before she strikes. Often, she does this by appearing to be lost or injured. When someone comes to her aid, she leads them to a secluded spot and strikes.

You can also alternatively make his claw/nail attacks (not just punch, slam, slap, etc), of the vampire, give a fast acting disease (contagion spell similar) as he is undead , despite not being a rotting zombie. For maintaining xp value and combat challenge levels if he fights unarmed. But the civilized ones would use weapons of course, and weapons would be preferred in general. So his physical attacks that were dangerous, still are.

AD&D 2e
https://adnd2e.fandom.com/wiki/Contagion_(Wizard_Spell)
This spell causes a major disease and weakness in a creature. The afflicted individual is immediately stricken with painful and distracting symptoms: boils, blotches, lesions, seeping abscesses, and so on. Strength, Dexterity, and Charisma are reduced by 2. Attack rolls are decreased by 2. The effect persists until the character receives a cure disease spell or spends 1d3 weeks taking a complete rest to recover. Characters ignoring the contagion for more than a day or so may be susceptible to worse diseases at the discretion of the DM.

D&D 3.5
https://www.d20srd.org/srd/spells/contagion.htm
Probably either Shakes, Red Ache or Filth Fever.

Something like the above but adapted, some may be too strong or have other characteristics that do not fit.

It didn't bother me much that the old school rules made energy drain such a scary thing. Beings powered by negative energy were no joke.

It forced players to account for how dangerous the encounter was going to be ... and plan accordingly.

The spells, Restoration and Negative Plane Protection come to mind. As do magic items like the Scarab of Protection.

Energy drain was certainly scary, but so too were the save-or-die effects from certain spells/poisons/etc. Or effects that drained ability scores, or effects that aged the character.

It may not have always seemed fair ... but with all of the avenues available to damage a character (outside of hit points), the previous editions definitely had plenty of ways to up the stakes as it were.

It was good that it was scary, as it made the creatures scary, but it felt nasty in a really petty way. Like a waste. The huge problem was and is that it wastes time in the real world and ruins the game in-game and off-game, and it distorted the reality in the fantasy world itself because it was a lousy mechanic that made no sense even in fantasy worlds. We need only keep the its a scary and dangerous monster, and keep things in a way that make sense and enhance the atmosphere. So we do not have to just remove actual game time and real time by losing levels.

Dying is not a problem, nor is even losing limbs, or aging unto retirement or being disintegrated etc though everyone will want to avoid it and find it scary. But losing levels, takes away the experience of playing in a way.

Level draining makes people feel cheated, betrayed, robbed, fucked over. Its not like the other things. I am fine with dying and being mutilated, but i am not fine with dying and getting from 10th level to 1 or even 9.

These are perfectly fine;
https://www.completecompendium.com/appendix/ghost/ "the mere sight of one causes any humanoid being to age 10 years"
https://www.completecompendium.com/appendix/banshee/ "A banshee's most dreaded weapon is its wail or keen... fail die immediately, their faces contorted in horror."
https://www.completecompendium.com/appendix/beholde1/ "Disintegrate (20-yard range)"
https://www.completecompendium.com/appendix/snake/ Amphisbaena "Victims failing to make a saving throw vs. poison when bitten die instantly."

These are not;
https://www.completecompendium.com/appendix/spectre/ "drains two life energy levels from the victim"
https://www.completecompendium.com/appendix/vampilat/ " Each tentacle strikes for 5-10 (1d6+4) points of damage and drains two levels of life energy from the victim." Yes, this is 2 to 8 levels per round. With Salient up to 5, its 20 levels.
https://www.completecompendium.com/appendix/vampire0/ "drain two life energy levels from anyone they strike" and with salient ability its up to 5 levels.

I find it striking that you are perfectly fine with four consequences that my players would not tolerate. Don't get me wrong; they don't like having levels drained, especially not at illithivamp rates, but they can recover from level loss.

Different tables are different!

You can die a heroic or untimely or stupid in a comical manner death, but having levels vanish is just miserable. Whether you live or die, it just sucks.

Dying and suffering and having scars is natural. And heroic, its part of the struggle.

A specter causes 1d8 damage per blow plus 2 levels loss turns you into a specter when you die. Its the dying and becoming a ghost that should be the scary part (not the "we will be robbed of real and fantasy in-game time by some bad monster rule"), so you can remove the levels loss that are incomprehensible mechanics and just take to 3d8 per blow and then you will get to die and become undead much faster. So this would be scary. The level drain mechanic should not exist.

The what exactly happens of the level loss via this manner makes no sense as well. Selective amnesia, faith loss, skill loss, this and that loss of very specific things in order to explain that suddenly the character actually lost Experience.

And what if the characters all get to level 4 from 11. How is that even remotely explainable. These are things that are not touched even with 10 foot poles.

Its just an amateurish rule-thing they added when they made the game that they did not bother to fix. The level loss mechanic does not generate anything other than a bad stressful feeling. Its just, "what the fuck? No."

Or even better, its like the "stolen valor" of gaming rules.