Traditional energy drain is a pain in the ass, especially with all of D&D's derive values. Get hit by a wight and you basically have to rewrite your whole sheet.
There are far kinder (but still scary) alternatives.
1. I think James Raggi suggested this once. Instead of losing a level, you loose all xp for that level. Easy math and it means it takes that much longer to level up again. This can be good for oldschool games where asymetrical advancement of PCs is common anyway.
2. Deal ability damage (con, dex or str) and/or reduce max HP instead. Again, much simpler and brutal, but not my favorite. Works well for many people though.
3. Negative levels. Instead of eating your levels, the undead gives yo a number of negative ones. The simplest form this can take is -1 to all d20 rolls and -5 hp per negative level. If you get as many as your regular level, you die.
4. My favorite one by far requires either healing surges, 5e's Hit Dice (with the slow recovery option) or my homebrew Class Dice. The energy drain eats those. It means simply that recovery time after fighting undead is longer, as characters feel sickly and weak and even if you kill the wight with most of your hp intact, you can't immediatelly recover and fight on. The attack rather literally drains your enegy.