Energy drain. Our favorite aspect of D&D! The evening is never complete without us losing half our levels to hoardes of the undead. I remember I was playing "Warriors of the Eternal Sun", a D&D game for Genesis (which had some serious gameplay issues...) and towards the end of the game my level 9 plus party was on the home stretch in the final dungeon, getting ready to bring Rad the Preserver on the scene to save the kingdom, and the party ended up having to go through a whole gaggle of wrights. It was funny because we'd killed all the wrights but afterwards my fighter was all the way down to level 3.
I remember back when I was in 4th grade I had encountered D&D for the first time, having bought the blue-covered rule book at a garage sale. I read about Energy Drain and I really didn't understand how it was supposed to work on an in-character level. If something drains your "energy" why do you lose EXP levels? If I came down with staphococus food poisioning and lost a lot of "energy" and needed bed rest I suddenly wouldn't start sucking at typing forevermore, or at writing research papers.
For literally decades the nature of "energy drain" was not adequately explained. Not in the D&D rule books, nor on the internet where lots of people would speculate but nobody came up with explanations that satisfied me. Even the old killer GM who introduced me to SR by killing my character every other session had a theory but it wasn't completely convincing to me.
Finally, though, Gary Gygax, who is practically the Chuck Norris of role playing, wrote "The Slayer's Guide To The Undead", in which he finally got around to explaining what the hell he meant all those decades ago when he first wrote up the "energy drain" attribute. His explanation was the first I'd ever read that really satisfied me. I felt that it was cool, and that it made sense. Now I'm a huge Energy Drain fan and I can hardly wait to get a level 10 first edition D&D character and then get him knocked back down to level 3 by Energy Drain.
Gygax wrote that undead creatures inherently have an intense jealousy and hatred towards sentient living things (another thing which I don't recall was ever well explained) and that on a literary or symbolic level certain undead monsters are symbolic to or related to certain negative or toxic personality aspects that can occur in people. As such they always seek to destory sentient life. Furthermore, the creatures with Energy Drain actually can go and negate a part of your life with their destructive magical negative life. When your life is negated whatever things you accomplished during that part of your life still exist historically but you have no mental or spiritual recollection of those events having happened. The impact of those life experiences on making you an empowered and self-actualizing individual has been cancelled out by the negative life energy possesed by Energy Drain creatures. So, for example, if a cleric had managed to save a single child from a ravaged village with healing spells, gained EXP for this, and levelled up, representing increased religious faith, but then the cleric were hit by Energy Drain and lost the level, the cleric would no longer remember the incident with the ravaged village and the child and would have completely and utterly forgotten his resulting heightened religious faith. The only way that the cleric would be able to get back to that level would be through creating new experiences and adventures to learn and grow from.
And that's why Energy Drain is so scary and cool. Because once you've lost all recollection of a certain part of your life there's literally no way in-game to get those exact memories back. (Except maybe a Wish spell). Imagine how sad and scary it would be if you'd gone through thick and thin with your adventuring buddies, seen horrors but also ethereal beauty in the dungeons, and long forgotten ancient treasures, but then you have an especially bad run-in with wights, they work you over, and you forget ALL of that. You can't remember any of your skills or any of your friends and those memories are GONE. They no longer affect you. If you're reduced to level 0 and become undead yourself it means that in addition to changing state to an undead creature you've also lost so much that you no longer know anything about who you are and the only thing that is left is a deep sense of jealousy and hate towards those who still do know who they are. That's some heavy stuff. It gives me goose pimples. It's a compelling concept with literary overtones (since if we look at how undeath can be symbolic of regressive or toxic living we can interpret a situation where an individual is pulled down to the same level of an antagonistic toxic individual) that would no doubt make White Wolf players gnash their teeth in rage since D&D isn't allowed to have anything cool or artsy in it.
In character it also gives characters more reason to be afraid of undead. It makes a lot of undead monsters less like cheap throwaway Halloween trinkets and more like loathsome creatures to be dreaded. After all, in character, who wouldn't be horrified on some level of potentially having their entire previous life ripped away from them and destroyed? Also, imagine how horrifying it would be to encounter someone you used to know and like, and see him irrevocably transformed into a hideous, hating mockery of everything he used to stand for? And there would be no cheesy made-for-TV-movie moment of redemption, no better nature for you to call out to, because all of that would be literally gone, cancelled out, no longer in existence. The person who used to be there would be gone just as surely as if you cut out his brain and mashed it beneath your boot but the body somehow kept living.
So, that's all. I just wanted to share my love of Energy Drain with you. Energy Drain was literally a concept that I hated all through 4th grade until very recently. Then Gary Gygax inspired a total reversal in me.