Zombie arm, which is occasionally dropped by zombies. It can be wielded directly as a club, or you can equip it with a weapon which gives you the ability to melee targets from a little farther away. Eventually rots away with the same timer as a corpse.
If you try to sell a zombie corpse to a shopkeeper, they get angry. Or maybe they merely won’t let you into the shop if you’re carrying a zombie corpse, the same way it works with digging tools.
Spell of necromancy that turns a corpse you’re standing on (or possibly adjacent to you) into a tame undead. Probably in the clerical school, though a case could be made for matter (and certain variants have a school of necromancy which this would much more directly fit into). It will work on corpses of undead that you have killed. Casting this spell would have some sort of penalty for lawfuls; there would be some sort of warning before trying to cast it.
You can normally only create zombies or mummies when an undead monster exists for the kind of corpse you are animating (i.e. a jackal can’t be turned into undead but a gnome can turn into a gnome zombie/mummy). For intelligent monsters that have no counterpart, the spell can create ghosts.
At a high enough skill or caster level, you can create skeletons from the corpse of any vertebrate, and maybe even vampires from humanoids. You can also create wraiths and shades from monsters that have no undead counterparts.
Zombies can’t open doors, but if there are multiple adjacent zombies on one side of a door, they can bust it down.
Whenever a zombie steps over a zombie corpse, it revives.
Zombies and possibly mummies have a new sound MS_GROAN instead of MS_SILENT. This would not do anything at the moment except give you “The zombie groans.” when chatted to.
Zombies destroyed by turning never leave corpses, or the corpses they do leave are incapable of reviving.
Not all zombies are hostile; only the hungry ones are. Thus, some zombies might be generated peaceful. However, a nonhungry zombie has a small chance of turning hungry each turn.
If a zombie finds a mummy wrapping, it will pick it up and transform into a mummy of that type.
Mind flayers have to grapple your head before they can successfully suck your brain (like in D&D), forcing them to spend a turn doing this. If you have a greased helm, this gives you at least one extra turn to act since they will not be able to grapple your head until the grease dissolves.
This would probably best be implemented by giving them a sticky grab or bear hug attack and ignoring the existence of their tentacle attack if they aren’t already grappling you (or you are being held by another enemy and thus can’t avoid them). Either way, checking u.ustuck is the key.
Possibly, they need to spend another attack getting your helm off, which will unequip it, though this will get weird if your helm is cursed. A cursed helm shouldn’t offer total protection against them. Possibly, they should be able to remove a cursed helm (like nymphs and foocubi do) despite its curse; this could be handwaved as them using their psionic powers to do it.
In variants that implement zombies eating your brain, it should work on basically this same system, where the zombie needs to hold onto you before it can eat your brain (and which also works that way in D&D).
Buff zombies, to make them more of a threat. Import FIQHack zombification (a zombie bite will place you 90-100 turns away from turning into a zombie, each additional bite decreases the timer by around 10 turns, but it will go away by itself if the timer doesn’t dip below 40); boost their difficulty, but keep their HD low; allow them to revive themselves less frequently than trolls, but also revive themselves whenever another zombie steps over their corpse.
Add a monster species flag for “zombifiable”, and track zombification or mummification as flags on an individual monster, removing the need for Z or M to be separate classes.
In variants with reviving undead, you can apply a weapon downwards at a dead zombie’s corpse to destroy it so that it can’t revive. This may take several turns.
Zombies always try to path in a straight line towards the player (assuming they can see or otherwise are aware of the player), and get stuck on terrain.
Change zombies’ and possibly mummies’ claw attack to a bite attack.
Buff late-game undead, because undead’s significance in Gehennom tends to be pointless since they aren’t scary by the time the adventurer gets there.
