There’s some undead that reproduce themselves, generally through the transformation of a humanoid; there’s other monsters which evoke a transformative effect on their victims through a curse, poison, or disease. Let’s talk about a few of them!
The heavy hitters here are the various lycanthropes, werewolves, wereboar, wererats and so forth. They reproduce without even having to kill their victims; a single bite, a failed constitution save, and the victim inherits the curse. That means that werewolves should ‘breed’ quite quickly indeed, since they can attack a community, widely inflict the curse, and retreat. That goes well with my immunity-to-regeneration change, since regeneration heavily rewards hit and run tactics. The only real downside is how long this takes to spread; the monthlong latency puts a crimp in their velocity.
For undead, the shadow is the canonical and, indeed, only example. They don’t eat or sleep; they can walk through walls, and they can disappear nearly every round. The turnaround on their touch is a few hours, which leads to a very rapid induction of a village indeed; there’s no limit to the number of turned shadows, and they’re unable to harm each other (well, maybe: it’s not clear that their strength drain doesn’t apply to undead, but I’d rule it doesn’t!). See previous article for a rework of the shadow that makes them exhaution-based, so that they have less of a special system.
That last paragraph may have caused some consternation. Aren’t I forgetting wraiths, wights, and vampires?! Actually, no, I’m not. Their victims do not become instances of the creator’s type; they’re specters, zombies and vampire spawn (respectively). Those types lack the ability to turn their victims, and so cannot start a plague of undeath! There’s usually the concept of dread wraiths, wight lords, etc; those are great ideas but would probably produce the base types (wraiths, wights), so their plague is still limited. That said, the wraith’s quick turnaround is terrifying, and since victim specters (like victim shadows) cannot harm their progenitor, the wraith can spawn indiscriminately, even if only a small number of spawn are controlled. This army can go through a small village before anything can stop it as a shadow army would, only slower, with linear instead of exponential growth.
There are other undead which do not currently reproduce, but really should. The sorest thumb are the ghouls; how am I supposed to have a Walking Dead-style plague of undeath if I don’t have a disease-based vector? See previous article for a rework of the ghoul that makes them disease-based, giving them a vector for multiplication.
The mummy is the most processed undead in the player’s handbook. It seems unlikely they could spawn by inflicted curse, but they should surely take their victims back to their sarcophigi and enact dark rituals to transform them into bandaged servitors? I’d assume that’s what mummy lords do via create undead but they don’t (canonically) have that prepared! To make it worse, they do canonically have harm prepared, and that’s surely too useful, and too much a part of their challenge calculation, to swap out! Luckily, we can freely swap prepared spells around between levels, given how preparation works in this edition; we can drop spiritual weapon in favor of create undead and then only cast one or the other in a given day. Now, as the spell is written, they can’t actually create mummies as a 6th level spell, but I think we can give the mummy lord a break (either as a special rule or by pulling the special monster types down in the spell). The create undead spell spends most of its time on the ghoul/ghast family, which (based on my desire to use them as a spread vector) is uninteresting to me. But it also gives me an idea of equivalence: as a 8th level slot, 5 ghouls = 2 wights; as a 9th level slot, 1 ghast = 1 wight; therefore, 6 ghouls = 3 wights = 2 mummies. Which means, if I’m willing to break things up a little, that as a 6th level slot it’s a mummy or a wight, as a 7th it’s 2 wights, 8 is a mummy and a wight, and 9th is 2 mummies and 3 wights. Now, I know why the spell doesn’t do this; wights can breed zombies, so limiting wights to 8th level prevents the spell from breeding quickly. But I say damn the torpedoes, it’s not like the mummies are gonna breed!
Anyway, a mummy’s touch leads to death (and it is a curse, not a disease), so they don’t breed, they (at best) construct new members, and are animated by external forces. Sort of like the skeletons, which also don’t reproduce. They’re produced via a low(ish) level spell. Is it possible that they’re only produced around a necromancer (or naturally occurring necromantic energy)? That interests me, because it means the relationship between skeletons and mages (or evil priests or other spellcasting necromancer-types) is very similar to that between zombies and wights. Of course, zombies can also be manufactured via animate dead, which somewhat ruins the symmetry, but that’s life.
The skeleton has always been the odd-one-out in this sense. It feels in some ways more like a construct, all clacking-bones and maybe-grandma-wouldn’t-mind-eternal-servitude. 5e gives them back that moral dimension (maybe other editions did too?) by making it clear what they retain of their former lives and their bloodthirst. But I still wonder; if you made them constructs (instead of undead; this would make them immune to clerical turning and un-hedge them from magic circle and so forth), would that be worse for the universe of the game? Legitimately not sure; the scarecrow is still a construct, despite its soul-bound and wicked construction.
Slaadi are perhaps my favorite. The game has long needed a self-replicating aberrant — the Xenomorph from Alien, perhaps? — and the slaad is very wiling to provide. I don’t care about their “I’m the paragon of chaotic neutral” thing, in fact, it’s the first thing I discard. But I adore their chestburster/chaos phage lifecycle, and all of the good ones can shapechange, hiding their infection for a good long while. Great for an invasion-of-the-pod-people scenario. They trigger on a pretty long cycle, though — months for the eggs to proc, days for chaos phage to finish. C’est la guerre.