#if anyone i know irl takes a look at my page i will actually combust

[blasvemous]

I have never felt more free to be my freaky self than on this site

[pomrania]

Setting people on fire in D&D

DISCLAIMER: DO NOT SET SOMEONE ON FIRE IRL WITH THIS ADVICE; IF YOU'RE GOING TO DO IT, I DO NOT WANT TO BE LEGALLY ASSOCIATED WITH YOU. Also, I don't have much of a background in chemistry; if I've gotten something wrong, forgive me and point it out so I can correct it.

This post is brought to you by @fuck-the-fairies, who asked for rules about being on fire; my ignorance on that subject and the possibly-tangent my mind leapt to; and @eyeloch, who is working on a chemistry degree (albeit not in the field of fire) and looked over some bits of this for me.

Now, despite what childhood edutainment implied, humans are not very flammable. Our bodies are mostly water, after all; and I'm going to be using that as a baseline for any other character race, because doing otherwise invites madness. Therefore, if you want to light someone on fire, you have to do something unusual.

People aren't very flammable, but sometimes they're covered in flammable things. Barring the presence of a massive fire source, I think that's the only likely way you're going to set someone on fire, without magic. The victim has to either be wearing ridiculously flammable clothing -- not even paper catches like that, I mean something on the level of guncotton -- or they've been doused in a flammable liquid.

The PHB (page 152) gives rules for splashing oil on someone, and says that if the target takes any fire damage before it dries, then it takes an extra 5 fire damage. That's okay, I guess, but it's boring.

First off, let's adjust the "takes any fire damage" so that it also allows for very small fires, that in themselves wouldn't deal any damage, to also trigger this. Like a lit match. Very minor change, but it plugs what I think is a loophole.

Second, would it ALWAYS require a ranged attack roll to pour oil on someone? In combat, probably. However, if the target is incapacitated or restrained or otherwise unable to dodge, I'd say you can just walk up and pour it on them. If you're upending a whole barrel of stuff onto a group of people from above, that's probably something that would require a Dexterity save on their part.

Now I'm not an arsonist, but I've read a lot of mystery novels. I know that accelerants are a thing that exist, and I also know that fire comes in different intensities. The stuff in the PHB, that's just standard lantern or cooking oil I assume. What if we used a different chemical compound to prepare the target? Same rules would apply for getting it on them, as with any liquid, but the effects are different.

Here's the variables we can change: how noticeable the liquid is, how long the liquid is flammable, what it takes to set a fire, how long a fire lasts, how much it takes to put the fire out, and what damage the fire does.

Let's start with the last one. There's how MUCH damage it does, obviously -- how hot the fire burns -- but you can also have stuff other than fire damage. Poison damage springs to mind, as a lot of substances emit toxic fumes when aflame. You could make a good case for radiant damage as well, although that'd likely be more mystical than physical. Psychic damage? Sure why not, if it's described as excruciatingly painful and attacking one's very sense of self. Extra damage types might or might not have an additional save associated with them.

Continuing with the reverse order, how much it takes to put the fire out. The standard rule, throughout effects which set someone on fire, is that it takes an action, by anyone who can reach the person on fire (including the person on fire). But what if it's sticky? Maybe it takes two actions. What if it can't be put out with an action by the person who's on fire, and it needs someone else? What if the helper takes fire damage from putting out the fire? Common sense would say that being submerged in water would put out the fire, but even without magic, common sense fails to take white phosphorus into account, which has its own oxygen supply and keeps burning underwater. You might need to remove anything that came into contact with the flammable liquid; let's hope the character was wearing a shirt.

I was talking with a friend who knows stuff about chemistry, and they mentioned the possibility of burning sulfur. Quote: "the combustion of the sulphur will form sulphur dioxide" and "this gas, upon dissolving in water, make sulphuric acid". I asked for clarification, and here's something REALLY nasty that's also backed up by science: if you use water to put out this fire, the person who was on fire is now NOT on fire, but they take acid damage. This is best paired with other things about putting out the fire, for maximum fun. (Also, apparently the flame would be bright blue so that's awesome.)

How long the fire lasts. As a general rule, intense fires tend to burn out more quickly, everything else being equal, I assume. If nobody does anything to put out the fire, and the character doesn't die, what happens? Unless you have some serious legendary magic with this, they shouldn't keep burning for eternity. Anything longer than a few minutes seems unlikely to my non-expert self. This is unlikely to be too much of a deal, but it's good to get down ahead of time because you never know.

What it takes to get the fire started. "Any fire damage" is a logical choice. But maybe something needs a lot of heat energy to ignite, and it only blazes up when the target takes a certain amount of fire damage from a single source. Maybe some things can or can't be ignited from just a match. 

How long the liquid is flammable. In the PHB for oil, it says that it dries after a minute. Some liquids would evaporate quickly, which apparently is called high volatility in science-speak; if you don't apply flame within the next round, then it's wasted. Some could be there, and a potential danger, until the target has finished a short or long rest. Anything longer than that is probably unfair and/or broken.

There was a really great bit in that one Sherlock Holmes movie with Robert Downey Jr, where a guy was set on fire and he hadn't noticed that he was covered in flammable stuff, because it was raining while he was sprayed with it. That's relevant to how noticeable the liquid is, and I suppose also how distinct and recognizable. In most cases, the target would notice that someone put a liquid on them. However, what happens next? Is it something that the target can rationally assume happened to be harmless to them, or do they realize "I'm about to be set on fire"? Is there an obvious smell? What about how it feels, is it greasy or like water, does it soak in or stay on the surface? Is it easily visible? Basically, is this a viable choice for stealthily assassinating someone with, because that situation WILL come up.

...I just realized that I forgot to talk about actually lighting the fire. Well, anything that deals fire damage, that’s obvious. But for just touching a match to something that’ll ignite with it.... 5E doesn’t have touch attacks, which is overall a very good decision to simplify things, but unfortunately that’s exactly what this would be, so I have to describe it. The most accurate way to put it would be like, melee attack using Dexterity as the ability modifier, and the target doesn’t add an armour bonus to their AC (but shields still apply). This might be too complicated for actual play, but it gives a place to start. It’s definitely a melee action though.

Outside of a combat situation, if you succeed in a Stealth check opposed by the target’s Perception, you’re all good and don’t need to roll, they don’t see anything unusual and you can set them on fire without making an attack. If they do notice, make an attack roll, possibly with advantage depending on how unsuspecting the target is.

Unless your group is mostly composed of people with a background in chemistry or noxious substances, it'll be the characters who have to figure out how to make or acquire those "enhanced ignition" fluids. This is most likely the province of alchemy, being the closest analogue to IRL materials science, and also canonically working with unpleasant and dangerous chemicals. Thus, any character with proficiency in alchemist's kit. (Wizards also feel associated with this stuff, but the wizard can damn well learn alchemy if they want it.) It'd be safe to tell them that such things exist -- hell, "alchemist's fire" is an item in the PHB, you're not going to break the game by going "alchemists have fun and exciting ways to set things on fire" -- but acquiring substances, preparing them safely, and identifying the properties of an unknown flammable liquid, that all would require some checks.

To say nothing of the fact that those substances are probably expensive and/or legally restricted. Unless "assassination by induced quote-spontaneous-unquote combustion" is common in your setting, that could be fun too.