So, starting this weekend I will be running the Order through an updated-for-Pathfinder version of the Tomb of Horrors. The real Tomb of Horrors, not the heavily wussified version released for 3rd edition. I've got a copy here and I'm going room by room to update the traps (in many cases increasing damage since Pathfinder characters have significantly greater HP) and make sure everything is just as unfair and brutal as it was before. Most things at least. I do feel compelled to have things "make sense" so there may be a few changes to how certain things operate. Hopefully these will not hinder the overall lethality of the adventure.
I've been wrestling with the problem of Acererak for a while. You see I wanted things to be deadly and lethal but the point of horrific puzzle-dungeons is that they do have a solution and Acererak seemed to be an unsolvable dilemma. Looking at his stats it seemed straight up impossible for the assumed adventuring party to beat him. I wanted to include him without weakening him (and certainly not by turning him into a fake skull construct) but at the same time it just seemed to be an impossible challenge for the Order. Well, I've made peace with his psychotic deadliness when I reread things and realized that Acererak wasn't actually a combat encounter, he was basically another of the dungeon's many traps and the best way to deal with him was simply not to "trigger" him. So that's fine by me.
But this did get me thinking...how can you overcome the demilich? So I decided to try and compile a list of different ways to deal with him. Some of the items here are my own ideas others have been found online and in many cases I thought of something amazing and original and then I looked online and realized people have been doing it for year).
Before I start...lets set some ground rules. First, we're only looking at this from the original context...that is the 1st edition of AD&D. These ideas are limited purely to spells and abilities available from that time and to party members of an appropriate level (as indicated in the original adventure).
Second, we're limited to the size of the original adventuring party in the Tomb (that is, no more than 20) and while not strictly limited to the equipment listed in the dungeon you are not allowed to go significantly beyond it (thus no +5 weapons for everyone).
Third, when you enter the dungeon you do not know Acererak's weaknesses. I've seen plenty of people arguing that Acererak isn't impossible for a well-prepared party, pointing to the use of Legend Lore and research as to how they got ahold of the demi-lich's weaknesses and prepared accordingly. However, at the start of the original Tomb of Horrors it's explicitly stated that information about the tomb and Acererak is exceedingly difficult and obscure, even for spells such as Legend Lore.
Fourth, leaving the dungeon and returning or months-long projects are verboten. This outside the scope and spirit of the original module.
First, let's address the Lame Solutions. Lame Solutions are those which might involve "defeating" Acererak but they ultimately serve no purpose. Remember, Acererak won't kill anyone who doesn't mess with him so really there's only two reasons to try and destroy him...one is to get ahold of those impressively valuable gemstones studding his skull. The other is to try and free the soul of a companion already consumed. Lame Solutions accomplish neither. The worst involve simply walling Acererak behind conjured walls of stone or iron, covering him with concrete, throwing the skull down a well, etc. At this point you might as well ignore it. Banishing the skull to another plane (easiest to do with a bag of holding or devouring) is equally pointless. The most common Lame Solution seems to be trapping Acererak in a bag of holding and throw him into the sphere of annihilation by the entrance. It does rid the world of what is presumably a powerful evil force...but then again Acererak doesn't seem to actually be bothering anyone (barring the events of Return to the Tomb of Horrors).
Well, what about some better solutions...
There's "standard" way, using his weaknesses against him. How best to learn his weaknesses? Well there are a few ways. The first involves using Legend Lore when you finally arrive at the skull-guy himself. This makes the spell much faster and more reliable and would (reasonably) give you some insight. However, there's no guarantee that what you learn will be helpful or it may be too convoluted to interpret. It's relying ultimately on the GM (and nice GMs don't run the Tomb of Horrors). A more reliable methods would be convincing the efreet trapped in the tomb for a wish to this effect (wishes simply to destroy Acererak are so laughably doomed that they aren't worth considering).
With this knowledge in hand defeating Acererak is doable...but still very lethal. Using a series of shatter or dispel evil spells will almost certainly end with at least one or two characters killed and the demi-lich targets magic users first...beating him with a weapon (while theoretically possible) has such specific requirements that it isn't worth considering (if you have a Paladin and if they pick the one uncursed sword out of the three on the floor of the tomb and if they successfully kill the demi-lich before he kills them). You could always try and nab the lich in a bag of holding, take him outside the tomb to get aid in killing him...but there's technically no rules in that prevent the demi-lich from draining souls from within the bag (this also kills most of the Lame Solutions). Again, it depends on your GM. A really evil GM will note that spells like Shatter and Forget allow saving throws...
Perhaps the best way is one of the first "official" kills for Acererak. In one of the other chambers of the Tomb is a crown and scepter which, when used in the incorrect way, will kill the wearer of the crown. By word-of-Gygax this is a perfectly acceptable way to kill the lich. Ergo the Gem of Cursed Wishing would likewise be a viable way to destroy the lich. Simply bring it to the crypt and make a wish (hopefully one so straightforward and minor that it doesn't get twisted to destroy you) and drop the gemstone right next to the demilich.
Another good one is the use of anti-magic shell. Simply use the Antimagic Shell, approach Acererak and pull his teeth and pluck the gems from his eyes. Sure you can't inflict hit point damage on him but he can't harm you either. You could also use a spell like Telekinesis (note the skull only triggers when touched or struck) to move him into the anti-magic chamber within the dungeon, rendering the skull harmless. This leaves the demi-lich "alive" but it does get you the treasure. An amulet of life protection could potentially confer a similar ability.
Likewise using TK or tenser's floating disk might be a way to get the lich to the altar room. There you could throw him into the glowing mists to turn him female and good aligned. The exact effects of this are unknown but it would certainly prevent him from being so eager to eat your souls.
Acererak is pretty darn invulnerable...but his immunities are limited to "harm" which is a pretty broad term but obviously not immutable. Given that the "crown and scepter" technique functions we can assume that harm does not include all effects absolutely. One possibility could be the use of the Magic Jar spell (undead are not immune to it and it technically involves no injury). Of course Acererak's saves are never given but it's safe to assume they're incredibly high (not to mention his intelligence and wisdom). Success is pretty unlikely but may be technically possible. Of course if it were my game I'd say a save on the demi-lich's part automatically traps the caster's soul in one of his teeth. A mirror of life-trapping likewise might function in the unlikely event the demilich fails a saving throw.
A self-sacrificing wizard could touch the skull and use teleport to go somewhere completely unknown, giving an 85% chance of instant death as he and the skull fuse with the ground.
Abuse of cursed objects is another technique although it relies on the PCs possessing them. A pair of eyes of petrification will turn their wearer instantly to stone. A Helm of Opposite Alignment will turn him good. A mirror of opposition summons a duplicate of the demilich which attempts to destroy it (and will vanish after the original's death). Dropping a robe of powerlessness over the lich is another possible solution. Of course these are objects that you simply may not have...but on the other hand if your GM is running in the tomb of horrors you've probably run into at least one.
My creativity is beginning to get exhausted. Anything else I've left out? Any other ideas?