Charisma indicates the maximum level for a character in any class. Monster powers that drain levels no longer drain levels- they drain Charisma. Charisma is now also a measure of your soul. When you run out of Charisma you die. Ignore Charisma damage when dealing with hirelings, followers and henchmen.Addendum: I have just looked at the monsters that do level drain. Seems in LL and S&W level drain is the standard undead big bad power. Vampires drink blood and I would tie blood to Constitution rather than levels. Perhaps they also have a level drain? I would be hesitant to give them both. I am fond of stat damage as a measure of capacity for damage, but not a fan of recalculating things as the stats drop, hence the last line of the above rule. I should probably make my next post a treatise on how stat damage can be your friend.
Addendum addendum: Thinking about it I would remove the vampire's level drain and replace it with the ability to drain blood on an incapacitated or charmed or willing target, and say they can drain their attack damage points of Con per turn. This fits with the Dracula image better. As for the shadows I would say they drain life force so rather than Str, Dex, or Con I would have them drain just Con. If I did this I would not require a recalculation of hit points and abilities. And replacing level drain with Cha damage helps you avoid recalculating also.