A fast and balanced alternative to standard Energy Drain attacks.
In the official rules, Energy Drain (RC/154) attacks reduce the victim’s experience level; while terrible in effect, this can bog down play as the target re-calculates hit points, THAC0, spellcasting capability, etc.
To streamline play, consider reducing CON instead. For every experience level normally lost by a successful energy drain attack, reduce the victim’s CON score by 1. For example, a wight’s touch reduces CON by 1; a vampire’s energy drain reduces CON by 2.
Per the rules (X27), CON loss is permanent and there is no save, but lost CON can be restored by any means that restores levels.