This article was first broadcast in Episode One Hundred Two on 1st January 2020.
Ryu (coughing): Why is there suddenly smoke everywhere? Ostron! What did you do in the workshop?
Ostron (muffled): This wasn’t me; I was trying to find a way to charge ROSTRO’s power cells faster and then heard all the coughing.
Ryu (still coughing): What is that thing on your face? And can you clear this out?
Ostron (muffled): One gust of wind coming up.
Ryu (deep breath): Much better.
(sarcastically): Oh look. We have one unconscious Lennon lying on the floor with a bunch of open potion bottles and a cauldron. Why am I not surprised?
Ostron (pop): Because your memory is still mostly intact.
Ryu: What do you mean mostly?
Ostron: Never mind. Lennon. Lennon!
Lennon: What? It’s not my birthday, you can’t make me!
Ryu: Okay…um…ignoring that for the moment, what were you doing?
Lennon: Oh…I was trying to figure out how resistances and vulnerabilities might work for different creatures based on chemistry…but grammar school was a long time ago.
Ryu: What does grammar have to do with chemistry?
Ostron: It’s another British thing. So can you walk us through your thoughts here, and maybe why we almost suffocated?
Lennon: Here, these were the notes I took.
Some of the standard statistics in D&D include damage immunity, damage resistance, and damage vulnerability. Now I say they’re standard because they’re always presented the same way in the stat blocks, but like legendary resistances or innate spellcasting, they don’t show up for every creature. In fact a lot of creatures don’t have any of them; they just take damage as rolled regardless of whether you hit them with a mace or a magical rock.
However, every source of damage in 5th edition has a damage type. Mundane weapons tend to do slashing, bludgeoning, or piercing damage, while natural and magical sources start adding in elemental damage like fire, poison, acid, and so on. Again, for most creatures that doesn’t tend to matter, but when you get into higher level creatures or ones with unnatural origins, like things coming from the Far Realms or the Nine Hells, what type of damage you’re doing matters a whole lot more.
Unfortunately for many players, the only reason it matters in most cases is because of resistances and immunities. Most 5th edition creatures that mention damage types in their stat blocks only do so to limit damage from some sources.
Slaads, aberrant creatures from the Far Realms, have resistance to any damage types that come from natural elements, like fire or cold, but beating them over the head or stabbing them still works. Elementals, on the other hand, tend to be resistant or immune to damage from whatever element they represent; hitting a fire elemental with fireball is a good way to waste a spell slot, for example. To make matters worse, all the elementals, as well as most fiends, have resistance to damage from the mundane damage types unless they’re magical weapons.
As mentioned, most of these resistances and immunities show up in higher level creatures, but many D&D and indeed RPG veterans have noted that 5th edition very rarely employs the vulnerability label.
In previous editions of D&D, when players encountered certain creatures, in addition to being able to figure out what not to use against them, there were also certain attacks or powers that would be more effective.
If something reared up and spat fire at you, throw ice back at at. If it’s a walking pile of soil or rocks, hit it with thunder damage. If it’s mostly wood, straw, or wrapped in cloth, set it on fire.
In 5e sometimes that approach works, and sometimes it doesn’t. If you run into a salamander, hitting it with cold will shut it right down, but throwing a cold attack at a fire elemental will only do normal damage. And things get even more uncertain if you venture beyond elementals and natural beings. It used to be automatic that if anything from the nine hells or the abyss showed up, you hit it with radiant damage to bring the pain, but now not so much. Relatedly, if you needed to settle an argument with a messenger out of Celestia, hurling necrotic damage was a good idea. Now it’s no better or worse than throwing a cloud of poison at it.
In light of how 5th edition was put together this sort of makes sense; with bounded accuracy in the mix, two of the easiest ways to make creatures more dangerous (increasing AC and increasing their likelihood of hitting) were taken off the table. Now the only things you can freely play with are HP totals and damage, and the amount of damage is limiting because reducing character to 0 in one hit isn’t usually fun for the player in question (also usually not fun for whomever’s playing the cleric and now has to listen to everyone begging for a heal).
Resistances and immunities, at a mechanical level, provide increases in difficulty that are conditional. The monster doesn’t have any more hit points than usual, unless players attack with something it’s resistant or immune to; then it takes longer to bring the creature down. It can provide a slight increase in difficulty without being as punishing as a blanket increase in the creature’s total HP.
However, only using vulnerabilities or immunities can sometimes feel like punishment to players. If you throw in a creature that’s immune or resistant to fire, the spellcaster that bulked up on fire spells might feel useless; even if they have a few other spells that aren’t fire-centric, chances are their big spells are based on that theme. This is even more true when you start throwing out creatures resistant to nonmagical damage, particularly if you haven’t been liberally handing out magic weapons. Suddenly anyone who isn’t a spellslinger is outputting a lot less damage than they’re used to.
Reintroducing more vulnerabilities can counter that, or at least make it feel less like a negative situation; instead of the monster simply eliminating a method of hurting it, it becomes more of a puzzle where the players have to figure out how to best employ the attacks that hurt more, while avoiding the ones that do less.
It can also make creatures more interesting and help with immersion. It makes sense to a lot of people that fire-based creatures will take more damage from cold-based attacks, and vice versa. That can even extend to basic logic. Skeletons, for example, are already vulnerable to bludgeoning damage because shattering bones makes them less effective. Similarly, scarecrows are vulnerable to fire because straw is extremely flammable. Reintroducing some logical vulnerabilities to creatures, such as making fiends vulnerable to radiant damage, or water elementals vulnerable to lightning, can help the creatures feel more unique and distinct.
You can even introduce more subtle advantages to employing specific damage types. Oozes tend not to be vulnerable to much, and often resist things like poison and acid. While it might not make sense to have cold damage hurt more, anyone who’s made jello or pudding can tell you the colder it gets, the less it moves, so imposing a movement penalty on oozes that take cold damage could make sense. Zombies may not be any more flammable than regular bodies, but if they’re wearing old clothes that catch on fire it could sabotage their efforts to find their targets, so imposing disadvantage could logically follow.
If you start doing things like that, make sure it has the effect on the players that you want. Throwing a bunch of thunder-vulnerable constructs at a group with a wizard who has thunder damage spells can make them feel like a superstar, but make sure they aren’t the only one who has any impact in combat.
Also, pay attention to the larger encounter balance. If you throw something on the field that’s vulnerable to acid and three different spellcasters start cantripping acid blobs all over the place, that’s going to alter how dangerous the encounter is, and how long it takes.
If you need to even things out in either case, mix in enemies who don’t have that particular vulnerability, or ones who are vulnerable to something else. A great trick if you can manage it is to throw a group at them where one half is vulnerable to something that the other half is resistant to, and vice versa.
If you want to take that even further, turn resistances or immunities into negative damage, and have the enemies be healed by whatever damage type they have an affinity for. For the superstar problem, mix in some more intelligent enemies who can recognize the rockstar damage dealer and will target them, forcing the other players to consider shielding their damage source or finding another solution.
No matter how you alter the monster’s preferences, hopefully they’ll give you some more interesting opponents to control, and for the players to fight.
Ryu: Okay, all of that makes sense but it still doesn’t explain the massive cloud we walked into.
Lennon: Ah, yes, well I was trying to find out if poison and acid would cancel each other out and…well something started bubbling, and then I saw this huge cloud of octarine coloured gas appear…
Ostron: That’s why the workshop has an air seal and these masks.
Ryu: Yeah, what is that, again?
Ostron: It turns out gelatinous cube ichor filters air really well. I encase it in this flexible shell and you can breathe through it.
Lennon: You do remember gelatinous cubes dissolve people, right?
Ostron: As long as the seal’s intact there’s no risk at all.
Ryu: Really? How long ago did you shave? Because your chin looks really red and shiny right now.
Lennon: All right so obviously the Oblex is going to want to discuss workplace safety with everyone, but right now I think some listeners want to discuss other things, so let’s get to the scrying pool… besides, I need some air…