Open Grave: Secrets of the Undead is one of my favorite volumes of addon material for D&D4. Not only is the premise of it delightfully hilarious (that it will teach you, among other things, the physiology of how undeath works in the D&D world, and justifications for it), but that they nevertheless manage to treat the subject matter so well (in my opinion).
I've decided to page through it again, and it's gotten me thinking about the mechanics of heroes in D&D. A lot of other, more high-stakes, RPG systems have rules regarding permanent disfigurement or mental damage from dealing with situations that are beyond the control or ken of the character, and even past editions of D&D had creatures that permanently sapped levels away from characters that they then had to earn back. D&D4 (as evidenced by the new "Fourthcore" movement) leaves a little to be desired in terms of high-stakes, high-impact encounters (in terms of theoretically-winnable-but-not-without-cost) sorts of situations.
Open Grave I think does a pretty good job at jarring me back into the notion of "oh yeah, undead are actually potentially really scary. There's something about the abstract, mechanical nature of 4e combat that ends up in making me a little jaded about the sorts of encounters that people can fly in to.
DM: You are surrounded by a ton of decrepit skeletons.
Wizard: Oh, I use Burning Hands.
DM: Ok, they're all dead now. again. For real this time.
There's something really existentially threatening and horrible about the idea of something that is dead rising to its feet again, charged with malign energy, that is trying to kill you too. And if it kills you, you may be dead forever. Or you may lose your mind and rise up again as one of them. I'm sorry, but the first time you walk into a room full of dead people that stand up at attention again when you step to the side and knock over a cup should be a really jarring situation, even among so-called "heroes."
I'll probably get in to why I love Heroic-Tier games much better than high-level games at another time, but Paragon- and Epic-Level undead provide a way to raise the stakes without necessarily getting the adventurers involved in a battle of potentially cosmic import. It's amazing how much godslaying there is in 4e; you begin to wonder why the gods bother to keep coming back.
But anyway, taking a page from Call of Cthulhu, I don't see why there shouldn't be a variant rule in place that dictates having encounters with something so cosmically wrong as undead that it is difficult to walk out of the situation in some way unscathed. These, of course, should be mainly boss-type encounters, but as a lot of RPG video games have taught us, optional mini-bosses can oftentimes be more frightening and more damaging than the main course of the game anyway. And this isn't even limited necessarily to undead, either. I see no trouble at all with having variations for aberrant creatures or demonic creatures, or other types of things that are belched up from the Abyss that have no business at all existing in the first place and should be destroyed as quickly as possible for the safety of the entire world. And they don't even have to necessarily be high level, either. Something the size of a pea could eventually feed and grow to threaten the entire cosmos.
But here's what I propose: rather than bringing back level-sapping attacks from previous editions (because leveling is so gnarly in 4e to begin with, unless you want to also hand out more experience things/create more experience-generating situations), to introduce variant enemy attacks that instead cause a permanent decrease in a core ability score. Liches are, after all, what's left over after a wizard or sorcerer transgresses the boundary of what is possible, so it would make a reasonable amount of sense that they might occasionally have the ability to grab hold of someone and literally sap the life from them, permanently causing a reduction in constitution, or strength, or something. Or some aberrant beast from some unknown plane that slips through a dimensional crack and literally destroys the minds of its victims, creating crazed half-dead animals in what were formerly commonfolk, and effecting a permanent loss in wisdom, intelligence, or charisma in a heroic character.
Depending on how nasty the DM wants to be, these effects could literally be permanent, or instead symbolize a sort of deep sickness from coming into contact with something so deeply "Wrong" as ordinary reality and the natural order struggle to again right themselves and restore balance (where in game terms the lost ability point would be regained after a level, a few levels, or at the next tier). Not only would these sort of profoundly weakening attacks encourage more tactical gameplay (like oh my god keep away from that lich, don't let it touch you), but would force contact-based players like fighters to have to potentially come up with alternate strategies, since just standing next to it and whacking on it while acting as a meat shield puts your character in even more mortal danger.
Lastly, it could be used as a sort of plot point for specific characters. Meaning, instead of the stat loss being literal, like they've been scarred or damaged physically in some way, that it was actually a blow to the psyche, and constitutes a loss of confidence, or faith, and that they therefore need to undertake some sort of personally redemptive quest, or join a religion (an introduction to multiclassing?), or accept some sort of stricture in order to get their abilities back. This last bit I got from Shadowrun, which I think despite some of its other faults, has a lot of really good mechanics in place to encourage good roleplaying. This sort of mechanic could also be used, in the D&D world, to explain why characters gain certain abilities and not others as they level up: this traumatic event at level 3 was why the character has the ability to do this daily at level 7. Adds depth to characters.
Finally at last referring to the title of the entry, it is a time-honored practice when referring to permanent stat loss to reflect the stat loss in a type of corresponding disfigurement. Why did the person suffer a point loss in Charisma? Maybe because that fire elemental burned his face with some sort of special hellfire. This is a readymade plot hook, as well. Perhaps over the course of their travels the party get wind of a mysterious sorcerer who has created a healing salve that can repair the damage from these sorts of attacks, but the path is fraught with danger and the salve itself is a king's ransom. And then there's the matter of the little rumor that the sorcerer himself may actually be a lich...
I love creating special magic items specifically for a particular adventure or campaign. Not only do I think does it make the adventure more immersive, but it also adds the level of surprise in case the players have been spending a little too much time reading the Adventurer's Vault. It is the latter reason that I also nearly exclusively create custom bosses for the climactic scene in adventures. Sometimes out of mixing together two or more enemies from the MM, or sometimes just out of whole cloth. It's especially joyous when I can successfully drop hints that misconstrue the true nature of the final enemy, so that the party is caught completely off guard. It would only add that much more depth to have an enemy that could potentially maim someone without having taken the necessarily precautions, which could even be hinted at all throughout the adventure leading up to it. Something to think about more fully, to be sure.