One of my D&D groups fought some zombies this morning. Fifth Edition zombies have a neat little mechanic that I’ve not seen before, in which they have a chance of not being destroyed despite being brought down to zero hit points. My players were fighting a handful of these guys, and the zombies were rolling really well (by which I mean I was rolling really well) on these saves, and several of them were absolutely refusing to go down. I started thinking that this might be frustrating my players, not knowing at all what was going on here as zombies stayed up, blow after blow. So I took a pause and explained to them the mechanic that was going on. Maybe that wasn’t the right choice.
The principal problem is that once a player learns how a monster works, they can’t unlearn that. By explaining how these zombies were staying up, I robbed them of the unique agony of trying to figure out what the heck was going on. And maybe they would have put it together eventually, or maybe not. This brings me to my next reservation.
I did a very poor job of describing visually for my players what was happening. I could have provided some clues that were apparent to their characters, but I’m still not sure how I could describe it that would gel with how the mechanics were playing out in this particular encounter, so I’m willing to forgive myself this point. The best I came up with in the moment was something along the lines of, “You feel certain that this blow has found the right spot to bring the zombie down, but it stays up nevertheless.” I feel very uninspired.
As a quick aside, in my games I won’t use the frequent piece of zombie lore in which one must attack their head. It just feels one-noted, in that most zombie deaths would then need to involve attacking the head, and it feels like this knowledge about zombies would have to be mechanically significant—people that know this about zombies should have an easier time killing them. But as I’m typing this, maybe each zombie has its own kill spot? I still feel uninspired by this idea though.
I felt bad about not giving my players the chance to figure out the mechanic on their own, but there’s a lot of variance in how it plays out, so it’s rarely guaranteed that a zombie goes down no matter what a player does. They could have gotten mired in the tedium of trying to figure out how CR 1/4 monsters die while they’re plowing through CR 5+ monsters. This lack of guarantee also means that there’s little agency given to the players by revealing the mechanic; it really just assured them that I wasn’t just making up which zombies died and which didn’t, and then provided them with some an insight they could use to increase the probability that a zombie would die when it should.
By the way, if you’re wondering what that zombie mechanic is (spoiler alert): when a zombie would be reduced to zero hit points, unless it was hit by a critical hit, it makes a Constitution saving throw with a DC of 5 + damage dealt, dropping to only one hit point instead of zero on a successful save.
As a final note, I am happy that monsters with such a low challenge rating made some kind of a name for themselves.
No Free Parking Money 
The mechanic is flawed because the behavior is not consistent with how we expect zombies to behave. We expect zombies to require a brain to function, even though their brain function is limited.
The zombies in 5th edition are different than the zombies we know and love – I think the answer as a DM is to lead the party to that conclusion before the encounter.
Perhaps several of the zombies go into combat headless, or without a majority of their body. This would, without directly expressing it, lead the fighters to question what was required to kill the creatures.
These type of zombies could be animated by some magic where their life-essence comes not from their brain but from a spell – it would be a fun boss battle to have to kill a warlock to truly stop the dead from rising again and again.
Either way I think the mechanic should be more nuanced – maybe each zombie has 1 unique chance to not die when reduced to zero health.
Sounds like an interesting learning experience – +1 to working with a questionable game mechanic.
I like Spencer’s notion of starting off the encounter description with headless (for example) zombies to signal to the players that these zombies are a tad different.
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I will quasi-respond to the main post with a related rant:
The root cause of the problem you experienced is the dichotomy between “hit” and “miss” as well as the interplay between “hit points” and “damage.”
A player is trained to believe that a series of hits that do damage will defeat an enemy by eventually eliminating its hit points. When the player does not experience this, it can be frustrating.
I propose fixing this problem as follows; I feel it it quite cinematic, and eliminates the frustration.
Zombies are clumsy, slow, and easy to make contact with during combat. What “really happens” in a zombie fight is that a blow struck against them is either a kill shot or a non-kill shot.
I would describe every attack that fails to destroy a zombie as an attack that makes contact but fails to destroy the zombie. I would still track hit point deductions to the enemy, but not tell the players. Thus, there is no “hit” or “miss” or “damage”–just kill shots and non-kill shots.
Eventually, when the zombie is reduced to zero hp AND fails it’s constitution saving throw, describe the attack as a “kill shot.”
With this plan, you still run the risk of frustrating players by wondering why the 19 they rolled (that is obvious a “hit”) did not reduce the hit points of the enemy. That’s their fault for meta-gaming, however, but is still a problem.
I would head this off by saying, at the start of the encounter, “Guys, these enemies can be defeated by you with the resources you have, however I’m going to be describing things cinematically to make the encounter more interesting.”