You're Doing Undead Wrong

Undead are more than acceptable targets for any kind of adventurer. We should not let this terrifying force of un-nature be just a different flavoured sack of HP that maybe is susceptible to holy water. It’s totally unfair towards the pure distillate of terror those monsters can be, if only you let them be as scary and invincible as they deserve!

Yummy.
Zombie from Resident Evil 2 Remake.

Ok, maybe I played too much Resident Evil 2. But that rant is not that far from the truth. I think that, rather than just have a weakness to holy damage or fire damage or the such, each kind of creature should fundamentally be unique, and understanding them to defeat them or work around them is a challenge all on its own and worth exploring. And once the threat is understood, seeing players interact with it in different kind of ways and seeing them find solutions to outsmart, avoid or use it makes for an hell of an interesting game. That’s an extremely valuable lesson I took from comparing RE2 to other zombie games. Did I already say I liked RE2?
Anyway, that’s how I plan to revamp undead at my table. I don’t plan to use all those rules at once, except when I will because I want players to hate me. Mix and match always makes for a good time.

Arise, my warrios!
Settra the Unperishable from Warhammer Fantasy.

The Restless
Some undead don’t die. That’s a fact. Whatever inhabits that weapon of putrescent flesh that’s shambling towards you can’t be just shanked out of this world. They are not like you.
After being defeated, Restless Undead come back up after d4*10 minutes to try and eat you again. They are the reason corpses keep disappearing from the dungeon.
Each Character can spend 10 minutes keeping them down, sitting on their shambling remains and hitting whatever tries to rise again. Up to 3 Restless per Character can be kept down, preventing them from coming back.
Each Character can spend d4*10 minutes to utterly dismember a single undead, destroying it for good. Alternatively, it can always take 10 minutes if you use something appropriate, like oil to burn it down, nails to crucify it somewhere, or a chainsaw to make sure it is appropriately scattered all around the room.

"I live... again!"
Tchernobog model used in Blood development by Monolith.

The Recreated
Some undead did die. And somebody around them was not happy with that turn of events. The scuttling claws and broken bones, gathered by madmen, found unlife anew. Hulking horrors stitched together and grafted to each other, in a perennial state of walking destruction, held in one piece by thin wires and threads of hate.
Recreated Undead have 3d4 limbs attached to their grotesque bodies. Half of them can be used to attack everything in range with a single Action. They gather 1d4 more each time they kill something and spend 10 minutes tearing the corpse apart. They can only do so with the corpses they kill themselves.
Instead of dying, they make a Save. On a successful Save, they lose 1d4 limbs. On a failed Save, their flimsily put together bodies fall apart like they were always meant to.

I've waited waaaaay too long to find an excuse to use this one.
The Curse from Dorohedoro by Q Hayashida. GO READ DOROHEDORO PLEASE.

The Revengeful
Some undead are simply dead. They linger on, and the violence that unmade their lives lingers on too. Ghosts of murders past and future, stealers of killing intent.
The Revengeful are incorporeal. They won’t attack unless they sense a killing intent, regardless of who it is directed to, in their area of action. This area extends 10 feet around them, and a faint glowing mist hoovers in it, like a stagelight.
When they sense the intent, they start audibly screaming. The mist becomes blood red. Your hands are actually covered in blood. You are actually covered in blood. The screams come from you now. They start haunting and attacking all those involved in the current conflict, inflicting constant damage as they ravage their victims with spectral teeth, claws and broken bones. They follow everyone involved until something drops dead. Those killed strengthen the Revengeful and sate its bloodlust for a day, giving it peace for a few hours.
They are spitefully condemned by themselves to relieve the key moments in their violent end. During this theatrical dance of death, they disappear every d4 rounds of reenacted suffering and silent screams, to reappear in the next spot of this macabre play. There are usually no more than 3 key spots/moments, that are forced to relive in loops for all eternity.
This ghastly, continuous rehash of their violent deaths can and should give out some important features and information on the dungeon, of course.

That's gonna take some time to clean up.
Screenshot from DAYZ.

HP, Speed and whatever else
Just make them fuckin’ slow and avoidable. One thing that the latest incarnation of RE2 taught me is that the undead are a fuckin’ phenomenal trap, and a not so great straight up fight. Fighting them is unreliable. Or at least, make most of them so. Exceptions are always a good time.
I make them move 10 feet per turn, inflict a good amount of damage (3 Afflictions, which is roughly a d6/d8 in B/X terms, probably not outright lethal but very scary) with a pretty high bonus (they reliably anything that’s not armored), and they go down as soon as they receive an Injury ignoring Afflictions (so they would have, like 1-2HD, but ignore all the damage that’s less than 4).
Being so slow and damaging, they are dangerous in numbers or in close quarters with little space of maneuver, which is pretty much how I think undead should work as interesting elements in a dungeon anyway.
I think I’ll have fun running them. I can’t really tell for my players, however I hope their screams and improvised plans to salvage the situation will be entertaining.

The Waxmaker: Beehive Incarnate

Between the Granite Halls and the Hillplains, a great road stands. Dusty feet of a thousand of Mankind’s finest and lowest alike have walked it, to meet the marvels of the Lord on the Hill and bring back news of such beauty. But sometimes dust gets muddy with water, when the wind blows from the Black Swamps to the north and brings dark clouds; and sometimes dust gets muddy with blood, when the highwaymen dare the Lords’ Cordon from the south and bring hunger and steel.
But neither blood nor water was found near the caravan. Only beeswax, and the footprints of many coming in and few coming out. No blood, no corpses and no signs of a fight, only molten wax and a swarm of bees hovering upon an abandoned wagon…

Unknown from the Internet.

Mud and Blood:

Zulal was born in the ghettos of the Ironmurk, the only city of the Scintillating Bay. He learned the value of life and death at an early age, and was forced into the Wilds almost as soon as he could walk. Nobody thought he could survive. He learned the art of bushcraft and learned to pick his fights. He learned the essence of death, and was touched by the Arcane Winds moved by a Feral Murkdweller in the middle of Nowhere. That’s where he noticed it. The winds didn’t touch him alone, and were not made by the feral. They were all-touching, and the fish-man was but a mere spectator pretending to be the director of the play. The bees showed the way the wind blew. The hive showed to truth. The hive was the truth.

He is always accompanied by The Swarm. The Swarm will always follow him closely. The Swarm cares for him, just like he cared for the Swarm when he gave it life.

The Swarm cares.
Wax Golem by epicprivate.

The Swarm:

The Swarm appears as a small army of melting wax men. This is not what the Swarm really is.
The Swarm is the mind behind the Wax Simulacra. It sees everything the Simulacra see, and care for the Hive as a whole. Every member of the Hive is protected by the Swarm. Outsiders are hated. Those that are hated are killed and fed to the Hive.
The Swarm doesn’t fight. It cares for the Hive and provides food for it. The Simulacra do just that: they jump at the enemy, absorbing it and slowly suffocating and crushing him to death. The suffocations takes 5 turns.
The Simulacra are weak to heat. Being near a strong heat source makes impossible for them to fully engulf a man. Attacking them with weapons has a 50% chance of making the same damage to the engulfed victim. Attacking them with fire sets them on fire, killing the Simulacra in 3 turns but risking setting the engulfed creature on fire afterwards.

He's watching you.
Wax Head from Dark Souls 3.

The Hive:

The Hive is a small house on an hilltop, kissed by sun and the moon, and just big enough for a small hunter to live in it.
Actually, that is just the entrance to the Hive. The real Hive is beneath it: the hill itself has been carved and emptied, and giant pillarcombs keep it in place. The hive is mainly inhabited by bees; other insects can be found but are not welcome. Most of the ceilings and walls are covered in beeswax, while the floors are usually covered in dead insects, rotting honey and loot from those unfortunate enough to have met the Swarm.
There are 3 main floors on the Hive: the wooden house, the Upper Cells, and Worker Bowels.
The wooden house is just that: a wooden house, cabin-sized, on top of an hill. Under a rug, a small trap door and a ladder lead down to the Upper Cells: a small maze of corridor, running around two main rooms: the Chamber of the Queens and the Observatory. Scattered around there are a few other laboratories and cages, of little consequence. The cages are always empty. The Swarm never takes prisoners.
The Observatory is a big circular room, with a large hole in the middle encircled by a wax railing. It overlooks the middle of the Worker Bowels, and is the only actual opening to it. There are no stairs and no rope to get down.
The Worker Bowels are a different deal. There are no stairs leading down. There are no real rooms, only ever changing walls of fresh beeswax. Most of the Swarm rests here. The pillars upon which the Hive is based on rest here. The treasures that Zulal and Rory have accumulated throughout the years rest here. The bones of the unfortunate victims of the Swarm rest here.

The Hive.
Houses on the Hill by Pablo Picasso.

The Road:

If found wandering around, Zulal will approach the party amicably, even if he is clearly a problematic individual. A beggar covered in rags head to toe, talking to insects and air. He will try to stop the party and entertain small talk, offer lunch, ask for help and anything else to keep them in place were they are for some time.
The Swarm will never be far, and will arrive very soon. Zulal never leads by more than half an hour of march away. Upon the arrival of the Swarm, Zulal will declare himself as That Which Was Born of the Wind, and will pretend surrender of body, soul and belongings of everyone else while the Swarm marches over consuming people alive.

A Magician in a Beekeeper’s Hat:

Zulal is a Level 2 Pack Rat, which spent both his Advancement Points on Spells. He is an Hexer, who is Doomed to see the Swarming Souls of the Self each time the Arcane Winds blow the wrong way.

Despite coming from Ironmurk, he is no Murkdweller, but a purebred Troll. He hates those slimy lizard with all of his darkened heart, and if there are any in the party he will immediately irradiate hostility and will make sure to gut him personally like the half-fish he is, after the Swarm has arrived.

He is a magician. His Grimoire is a small collection of wax candles, held together by an iron spiky ball that impales them all; they don’t burn and are not consumed by heat. It contains 3 Spells: Wax Simulacra (turns bees into wax golems), Luciforms (draws flames away from their source and around the caster), and Goliath (turns the caster into a crazed Xenomorphesque monkey).

Like this but with troll tusks.
The Pain from Metal Gear Solid 3: Snake Eater.

He will always cast Wax Simulacra each day just before preparing his lunch, so he functionally has only one Spell Point each day (unless caught before 11 am). He will avoid direct confrontation and will avoid using his last Spell Point unless really pressed to or unless meeting a Murkdweller.

When his hand is forced, he will cast Luciforms to catch all the flames that could menace his Swarm and then order the assault. He will use the flames to shield himself from any who get too near. If he is caught away from the Swarm, caught in the morning or meets a Murkdweller, he will cast Blackblood and rush towards his enemies holding a Hikkalan Sword (all Hikkalan weapons are decorated with removable boneshards, which remain in the hit target making wound treatment harder)  which he looted from an exotic caravan. He never wants to resort to Goliath, since using it reminds him of the circumstances where he found it, among the crazed Goblins of the Black Swamp, worshipping their cruel Orc-like shaman as a God.

After the fight, if there are any Murkdwellers, he will take out his ritual kukri made from a single metal bone of an Iron Leviathan, and gut them on the spot, making sure to scatter their entrails around as much as possible. He will try to do this while they are still alive, if possible.

Fucking fishman.
Kuo-Toa from the Forgotten Realms.

Magical Engines, Mechanical Arcanas:

Hexer (Occult Profession):
Hexers use Occult Catalysts to control the flow of fate. Creating a Catalyst takes 10 minutes. Catalysts can either be Totems or Dolls. Both can either reduce or increase the Difficulty of any check by 1. Totems work in a 30’ area, affecting everyone (friend or foe); while Dolls only work on a single target. Dolls require a small quantity of blood from the target to work, which can be gathered with an attack (inflicting 1 Affliction). Catalysts always weight like 1 item.

The Swarming Soul of the Self (Doom):
Your body numbs and your vision becomes multi-faceted, as if you were starting to see from a million different eyes. All over your face cracks open, like hornets nest, from which a swarm of insects flies out, holding your soul inside them. You control this swarm for the next 1d20/2 (d10) hours, during which the swarm slowly disperses as you feel your body once again. After the Doom is over, you retake control of your body. While in swarm form, you can’t carry more than a single Item worth of weight, and can’t make complex actions like using a weapon or picking a lock.
Scar: the cracks from which your soul swarmed out remain on your face, and hornets can sometimes be seen fling out of them.

The only good fishman is a gutted one.
Scene from The Simpsons.

Wax Simulacra (Spell):
A single insect is focused on by the caster, imbibing the power of the Swarm in it. The insect bursts, leaving a wax cocoon behind. The cocoon grows into a humanoid shape in the following hour, becoming one of the Swarm. It moves with the will of the Swarm, and is not bound to the caster.
It can be empowered, to have the cocoon grow instantly.

Luciforms(Spell):
The caster chants the song of Fire and raises his hands, causing all flames in a 500’ radius to raise themselves, flying a dance around him. All possible fire sources (including fireplaces, torches, and lanterns) in the radius have their fire removed. Each fire starts circling around the caster, who can at any time snuff one out or throw it towards any enemy. Throwing it requires an Action, any number can snuffed out with a single Action. After 10 minutes, all the remaining fires go out on their own.
It can be Empowered, causing 3 flames to rise from each fire in the area of effect, and allowing the raised flames to last 8 hours.

Goliath(Spell):
The skin of the caster turns dark gray, while his veins blacken and swell as if they contained liquid charcoal. For the next 10 minutes, the Strength score of the caster is 18. Every time he receives any kind of weapon damage, a gush of blackened blood sprays outwards all around him, inflicting equal damage of acidic nature to anyone in a 5’ radius.
It can be Empowered, growing black spines and a dark carapace on the caster. It now counts as having Heavy Armor without any of the maluses associated to it (and splashes acidic blood as if it was naked, even if he doesn’t receive the same damage).

Really, FUCKING FISHMEN
Murloc Raid art by Blizzard.