For background to this post, see the following:
Patrick Halter's post on converting level drain to GURPS.
My response, where I discuss level draining and what I think is the folly of converting rules to rules.
Patrick Halter's response to my ideas.
Okay, all up to speed (or you just don't care)? Let's go.
In my own Dungeon Fantasy game, I use undead from GURPS (especially the excellent GURPS Undead), Rolemaster, and AD&D, as well as from other sources (video games, my own crazy ideas, etc.)
Most of them are fairly straight-up conversions, since the effects are "game world" effects (damage, death, disease, aging, fear) and not "game rules" effects (loss of temporary constitution, level drain, or status/stat change - Rolemaster, AD&D, and Bard's Tale respectively).
For the level-draining AD&D conversions, here is some of what I've done.
Mild Life Drain
My wights are AD&D wights, but instead of level drain I gave them a combination of a claw attack, paralysis, and a followup drain of fatigue. I felt this was a scary combo (and it's proven to be so) but not unfairly punishing. Here is it statted up for DF:
Claws (14): 1d+2 cutting plus 1d FP followup.
Paralyzing Touch (HT): Victims touched by the wight are paralyzed for (margin of failure +1) minutes; cosmic, ignores DR.
The basic effect of this is that if the claw attack penetrates your DR, you also lose 1d FP; either way if you're struck you may suffer paralysis. Wights also have Infectious Attack, so if you're slain by a wight you'll rise as a wight later on. More powerful wights are easy - up the base damage, give them more FP drain, reduce the resistance to the paralysis, and/or change Infectious Attack to Dominance, so they can rule over their created allies.
Why is this scary? You might get paralyzed, so you're a bad roll away from out of the fight. The claw damage isn't weak, either, and the dual track losses of energy (FP) and health (HP) means you're fast-tracking towards death and losing the resources you most need to counter them.
That's pretty tame as far as permanent damage goes, though. Sure, if you're dead and your friends lose your body it's bad. But win the fight and you'll just need to get healed up normally - the Infectious Attack isn't the werewolf/vampire kind that infects those who survive (although that's a nasty variant too). So here is an undead that does something much nastier - it inflict lasting harm until a power cleric removes the effects.
Lasting Life Drain
This attack is from a ghostly undead that shall remain nameless until my players fight them.
Death Touch (16): 1d toxic damage; cosmic, ignores DR. Cannot be parried or blocked.
Life Drain (Special): Resisted by HT, minus 1 for every point of damage inflicted by Death Touch. Failure means -1 to ST, DX, and HT indefinitely - it can only be cured by Remove Curse.
In DF, Remove Curse takes a Power Investiture 5 cleric (out of a 1-6; the base PC clerics start with 3), and it's not terribly cheap in mana to cast, so it's not an easy fix by any means.
If you prefer to make these even nastier, add this to the end of "Life Drain."
" Critical failure on the resistance roll lowers one of these stats permanently." Or just put "One Try ever" as a limitation on Remove Curse for this particular effect, so only one priest ever gets a shot at it. The priest probably won't roll a 17 or 18, but if it happens . . . you're screwed permanently barring Great Wish (possibly).
You can easily make these Afflictions - either a condition (Heart Attack, Coma, Seizure) or one that grants a disadvantage (Epilepsy, Hemophilia, Wounded, or Chronic Pain leap to mind) or a stat loss. You can make them all pretty nasty and permanent (well, the disadvantages and stat loss) if you really feel like you need to put total fear of these undead into your players.
Costing these up in GURPS isn't difficult, but I didn't bother because they are monster powers and I don't care how much monsters cost to build. They should be possible to build, though.
Anyway, those are two actual undead "life drain" effects from my GURPS Dungeon Fantasy game. I don't like ones you can't fix - yes, I know AD&D level drain was fixable, but we didn't let people just go get Restoration in the good old days. So mine tend to be "lasting" and not "permanent." It's there until you get something done about it, which won't be free or easy, but it can be done. Couple that with infectious attack and a tough monster and they are genuinely scary in the same way level draining undead are, without needing to try to convert the mechanic of another game to the mechanics of GURPS.
Oh, and Merry Christmas!