Perhaps the most ill-named monster in all of the D&D lexicon is the demilich. Even a cursory glance at the Monstrous Compendium entry for this fiend is flaccid. It basically says a ton about liches themselves and then concludes with “Oh, by the way, there is this thing called a demilich that is way worse. Way worse. Like, turn and flee at the mere mention of it. Seriously. This is the monster at the end of the campaign that rises up and slaughters everyone with some kind of demonic howl. So a lot worse than a lich. Thanks bye.”
As much as I love the “van Richten” guides that helped define the horrors of the AD&D 2nd Edition Ravenloft setting it was the “Guide to the Lich” that truly infuriated me in how it dealt with the demilich. It got something like a half chapter that essentially says the same thing: “This thing is designed to kill characters. You will die. Horribly.”
There is naturally a good reason for the vagueness surrounding properly defining the demilich. It is even perfectly reasonable to leave the creature’s powers somewhat nebulous. You see, a demilich isn’t so much a lich as it is a sort of demigod. It is such a uniquely powerful force of pure evil that no gaming world should have more than two or three of the things lurking about. Even so they ought not to interact with each other because if they started a fight with each other entire nations would be obliterated, and even more disastrous would it be should they get along. They have given up having a mortal-ish form in this life and spend their time roaming other planes of existence, or lost deep in the troubled meditations of the insanely wicked. They may or may not continue to scheme vile and odious designs against what they perceive as a weak and squishy world but nothing should be taken for granted. What one must know, and never forget, is that a demilich is not a creature that will be brought down in one game session. It might not even go down after a years long campaign. Destroying a demilich should be considered as daunting a task as knocking Odin off of his throne or besting Hercules in arm wrestling. Unless the characters are a minimum of level 15 and have everything going for them they are going to need a lot of magical gear and NPC allies to pull off this sort of feat. Not just normal magical gear, either. We are talking relics and artifacts. You really need the mindset that you are going up against the biggest and most terrifying enemy possible.
With all of this in mind it is important for every aspiring Game Master to put a lot of thought into their own personal demilich(es). Give that wicked so-and-so every ounce of your creative energy. Make it feasible for it to have had a hand in every villainous deed aimed at the players, especially if they have been playing for a long time. Every time they jump at a shadow, play it up. Later on they may decide the demilich has been plaguing them the entire time. Because it probably has. It is the Sauron of your game. Incorporeal, yet ever present. Undefinable, yet strong and cunning. Short of entering actual deities into your story the demilich is very likely the most powerful being you can toss at your players. So play the long game. Let them determine over time what they are up against. Spend months laying out the clues. The time will come, I assure you, when they will completely lose interest and your time will have been completely wasted. But you have a really awesome imaginary friend and it knows how to scream.