Sometimes you need an undead army that isn't just the restless dead, the shambling hordes of a mad necromancer. Sometimes the undead are an unkillable obstacle that must be handled with overwhelming force, finesse, or guile. The necromancer or the death artist is generally the obstacle. But sometimes you need a set of undead who are the problem in and of themselves. I love stories about skeletons who put themselves back together, or zombies who continue to crawl when they are destroyed.
I want to focus in the idea of the zombie, the ones that explode, leak poison, or keep crawling. Similarly I want mummies who can live through anything, except for their heart being destroyed, especially when their heart is stored somewhere outside their body. Or skeletons who will gather bones to reinforce themselves, and cannot die. These are the kinds of creatures who are supposed to be overcome as a force of nature itself.
The zombie is a creature of rot and decay. The purpose is to show the danger, rot, and horror of a situation. Typically these are old bodies, ancient bones and rotten flesh. Their creators place surprises, leeches, poison, acid, or rotting sickness inside them. The kind of thing that foretells the rot of a kingdom, or its lord. Most zombies can be killed with a mortal strike to the head (an incredibly hard thing to arrange without a bow or a few friends), but others will keep twitching and attempting to kill with just a single arm remaining in a mockery of death.
While the mummy is not really a historical monster, it has a central mythos. The mummy is invincible, impossible to kill, instantly regenerating, except from the horrors of fire, the mummy is inevitable. The only weakness it has is its organs, kept outside the body. The real value is that it's an impossibly strong enemy who can't be passed without a tactic. Most of these are set as watches. It's rare to encounter the mummy of ancient royalty, and far more common to find the remains of a Yuan-Ti sorcerer who was interred and chanted to keep watch upon their tomes of lore.
The skeleton rising from the teeth of dragons, or from piles of bones, and grasping with unthinking and unfeeling claws is the most normal of these creatures. These are the tools of sorcerers and the desperate for power. The difference from the ordinary restless dead is just that they are more tenacious.
Unkillable: All of the undead cannot be killed without something special. They do not suffer wound penalties, at all. If they receive a mortal wound to a piece of their body, they cannot use it, and that piece is cut off permanently (treat as the Missing Limb trait). Most of these creatures have no use for their heads..
Mindless: Mindless undead are not properly alive, merely puppeted by foul magics. This means they do not behave like normal people, ignoring motivation. They cannot be persuaded or mind controlled. They do not have a steel score and cannot feel fear.
Full of Leeches: When struck, a creature full of leeches begins to leak horrid diseased creatures. When struck with a weapon these horrid creatures spray outwards. The striker must make a speed test with an Ob of the Wound dealt, or be covered in leeches. These leeches can be burned off, but if left on for more than a minute they begin to drink blood and spread disease. The blood is negligible, but the creature receives an Infection, raising the Ob of Forte (see the Anthology).
Poisonous Breath: When struck in the head, or having a chance to breath on a character (strike in combat), this creature can spew poison in a noxious cloud. This poison causes a Midi wound to anyone who breaths it in (Ob3 speed test for everyone engaged with the creature to avoid when striking it, or can be Blocked or Avoided if the zombie is intentionally attempting to poison). This wound counts as a Burn for deciding treatment and deterioration (deteriorates in 1d6-1 days and can deteriorate 5 steps, all the way past mortal).
Acid Blood: Creatures with acid blood spray acid when struck. When striking them with a slashing or puncturing weapon, dealing a light wound or greater (something that bleeds), means that anyone engaged with the creature when struck must pass an Ob3 Forte test or take a wound equal to the Forte of the creature with acid blood, armour applies (but is damaged on 1s and 2s).
Canopic Jars: A mummy cannot be harmed while its canopic jars still survive intact. However it cannot use limps that have been mangled beyond recognition. When a part of a mummy is Severely wounded or worse, it cannot use that part (treat as per the Missing Arm, Missing Leg or Blind trait, depending on if it's the arms, legs, or head). Keep track of where these wounds are, since at the beginning of each exchange (during the engagement), the Mummy begins to regrow, reducing the severity of a wound by one step, and it may grow back part of its body. They can still regrow if they receive a mortal wound, despite the Unkillable trait's permanency.
Pile of Bones: A skeleton may use a bone to replace any of their body parts, discarding the old bones and using new ones. This is the only way they can recover from wounds. Skeletons also receive half again damage when struck by a bludgeoning weapon, but no damage from piercing weapons. Meaning a Mark hit from a Mace doing B6 damage wound now deal B9, but a severe hit from an elven bow wound not harm a skeleton at all.
Destroy The Brain: When a zombie's head is a destroyed by a mortal wound, the zombie stops functioning and is destroyed.
flammable: Mummies, when struck aflame, will burn. This will deal a B6 wound to the mummy every round until the flame can be extinguish. Mummies will eventually burnout and then regenerate if their canopic jar is not destroyed while it burns. Creatures who touch the mummy while it is burning take the same wound. Creatures who get close enough to Fight are at a +1Ob penalty due to the heat.
A Burning Wheel Monster Every Week