I don't think HackMaster will have a level drain effect, but I've always liked it (as a GM). In the end, level drain was one of the few things that old school players feared (that and shit that turned you to stone). It was utterly terrifying and it makes a creature significantly more fearsome.
The problem I've got with it though is the amount of paperwork it always generated. To do level drain justice you would really have to track everything so you could undo it. In the older games it was easy, but as more and more bits and pieces came into play, it became too cumbersome.
Recent games have replaced level drain with a stat drain. Fuck that. Talk about book keeping! Adjusting complex combat stats on the fly sucks, especially while engaged in combat. It works great in computer games, but is a drag in table top games. HackMaster has CON draining critters which isn't as bad, but I still don't like it that much. Effectively, all it does is make a different set of hit points.
Energy Drain is Awesome
At some bogus point (probably somewhere in 2eAD&D) level drain started getting called energy drain. After a bit, though, I decided that I liked the term because it fit with a new way that I can make some of the old monsters in HackMaster seriously badass again.
When an energy draining critter hits, the character looses experience points. Metaphor this all you like into essence being drained, memories of skills being sucked out, or what-have-you. The result is that characters are dragged into a state where they have to work harder to level up. The important part here is that a character doesn't lose any levels - just the EP. Normally if you get level drained, you are double screwed because you probably aren't tough enough to deal with things any more - a ghost zaps your cleric, your turning ability gets worse, making the ghost more likely to stick around!
A character dropped to 0 EP simply loses the will to be an adventurer any more unless they were 2nd level or less - they can make that EP loss back up pretty easily. A 10th level fighter dropped to 0 EP though is just going to throw in the towel and become a farmer.
Experience Drain Number Crunching
Multiply the damage by the Will Factor of the undead, then x 10. Too fiddly, round the WF to the nearest multple of 5.
- A wraith has a WF of 18 and does 1d8 damage - so that would be between 180 and 1440 EP lost per hit (or 20 - 1600 with the simplified method)
- The barrow-wight has WF12, damage of 2d4: 240 - 960 (200 - 800 simplified)
- Vampires enjoy WF 20 and d4+4 damage and a bite for d6+4: a range of 1000 - 2000 EP per attack