On-the-Fly Crafting

Question

I do not believe there are any 5e rules for this, so this is as much asking for potential houserules as anything else. If there is an official rule somewhere, by all means, please point it out.

Let's say a PC is in a dungeon and would like to craft some kind of explosive arrows by rigging an arrow with various materials, right then and there. It's not a matter of using the 5e Basic Rules for crafting, because it's quick and dirty and he does not need a whole day to do it.

What kind of skill rolls would be involved in this?

Or are there any skills? Should it just be untrained stat checks? If so, how many and what kind?

Update: Specifically, I'm interested in which skill(s) or stat(s) would apply. Or perhaps an additional skill is house-ruled in for this purpose. The type of crafting is essentially akin to what a Tinker Gnome would do, cobbling together things to create something greater than the sum of its parts, but without that ill-fated race's somewhat... dramatic results. A DC would be determined per the difficulty of the item being created. Perhaps success or failure on various rules results in different degrees of success or failure.

Or am I completely overthinking this?

Answer 1

I think SevenSidedDie's response covers the basics pretty well.

On the specific question asked, if I where the GM, my ruling for this action would be (I am considering L1-4 characters):

1. Make an Intelligence ability check DC 13 during a short rest.
2. Spend 2d6 G worth of materials.
3. On a success, creates 1 explosive arrow. It must be fired within 1 day or it looses its effect as the components dry up. You can create and safely transport a number of arrows at a time equal to your Intelligence modifier.
4. Upon attacking with the arrow, the target and every creature within 5 ft makes a Dexterity save DC = 8 + Int modifier. Each creature that fails takes d10 fire damage, or half damage on a success.

Aside from following the mental process described by SevenSidedDice, I made these specific choices along the path:

1. The effect is kind of in the same tier as a level 1 spell.
2. The material cost and expiration date on the arrows ensures that casters with a similar effect are not overshadowed by it. The 2d6 is just a ballpark estimation, could be too much or too little depending on your style of play.
3. For more potent explosive effect I would add a higher DC to the crafting and much higher cost, but try to limit the damage and effect to something appropriate to the level of the characters.

Answer 2

Are they just putting together things you already understand well?

For example, suppose they have a bomb from the Dungeon Master’s Guide, and they want to launch it via bow and arrow for extra range. Just let them do it. If you think it’s a cool idea with no real drawbacks, just use an ordinary bow attack with bomb damage. If you think it would be difficult or dangerous, perhaps make the attack with disadvantage, or disallow proficiency bonus, or both.

Are they trying to invent something entirely new from raw materials?

In this case, they’re effectively designing and crafting a new magic item. The Dungeon Master’s Guide has rules for this, both designing the item, and allowing players to craft it. Magic item crafting is not a quick-and-dirty thing. It requires researching a formula (either finding it, or inventing it yourself) and then a long time crafting it so that it’s actually effective and reliable and won’t kill you. This has the benefit of giving you time to decide whether it’s actually a good idea and something you want in your game, rather than making a quick decision on the fly, and it avoids the problem of overshadowing spellcasters through ad hoc invention. It’s also quite reasonable to require a long time for any effective results, as discussed in “How to deal with player MacGyver-ism?”

Do the players want to do this sort of thing on a regular basis?

If so, you may want to create a new class specifically tailored to this sort of invention. You can use the spellcasting rules to represent super-invention as spells like flame arrow, and then create a tinker-flavored class around it. The Dungeon Master’s Guide has guidelines for creating new spells and classes. This will give players a long-term outlet for this kind of creativity, without putting you on the spot for ad hoc invention rulings over and over again, and again it helps to maintain balance between inventors and other spellcasters.

General guidelines

The D&D 5 approach to invention is to make it a downtime activity. The Dungeon Master’s Guide gives guidance for creating new items, spells, and classes, but it expects you to do all of these things between play sessions. That way, you don’t need to make hasty decisions about potentially unbalancing inventions, and you don’t put the game and all of the other players on hold while you work out the kinks. For very simple and straightforward things, however, you can ignore all that and just let the players have fun. Still, it’s important that you have fun too. If the pressure of ruling on these on-the-fly creations is too much, then push it to downtime, as the game has a lot more support for you that way.

Answer 3

You don't need homebrew for this, because 5e already accounts for using ability scores for things that aren't explicitly covered in the non-exhaustive list of skills (appropriately enough, this is first touched on in the "Using Ability Scores" section of the Player's Basic Rules, where each ability non-exhaustively lists examples of Other X Checks that the DM could call for).

You make a ruling instead, and move on with the game.

For those not familiar with such improvisational freedom in the DM's role, here's a basic breakdown of how to make a ruling for this kind of action:

1. Pick an stat that is relevant to what is being attempted and how it is being done.
2. Set a DC for the attempt, based on how hard it would be for an average untrained person to accomplish given the current circumstances, materials, rush the PC is in, etc. (You can refer to PBR page 58 for a list of DCs by subjective difficulty.)

3. Yes or No: does the PC have a relevant skill?

   • If no, skills are not relevant. Have them roll their ability against the DC.
   • If yes, they get their proficiency bonus as normal.

4. Have them roll against the DC, adding whatever relevant modifiers (e.g., proficiency) to the roll.

DMs used to making rulings like this (read as: "using the existing rules to handle an unexpected action") don't consciously go through this whole process, but it's an accurate description of how it effectively works. In practice, and with practice, this becomes second nature and doesn't have to be explained and laid out like this.

For example, I would have the character make an Int check, since jerry-rigging an explosive arrow relies mostly on coming up with an idea that works and being clever enough to figure out how to execute on it properly.* A relevant skill† might be Improvised Explosives, Knowledge: Explosives, or even I Have A Cunning Plan (though that perhaps would cause them Disadvantage...). I'd pick a DC of, oh... let's say "Hard" (DC 20), and inform the player of such. Then I'd ask them if they'd like to proceed, and have them roll if so.

But that's just how I would make a hypothetical ruling. I might make it differently, based on the exact circumstances and methods involved at the moment. The point is, you can't really do it wrong, except by letting it bog the game down. Make a ruling that's good enough, and move forward to get to the fun stuff. Consistency evolves naturally as you establish a history of rulings and they guide your feel for how to make new rulings.

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^{* If they were also trying to do it in a rush or otherwise being reckless, I might have them make a Dex-based saving throw (in addition!) to not blow something up, but skip that if they were taking time to be careful.
† Skills are usually listed with their associated stat, but they can be used with other stats if the circumstances make sense (e.g., Sleight of Hand is normally (Dex), but could be used to judge a friend's outfit for vulnerability to pick-pocketing using Int). As such, I haven't bothered to list a stat with these made-up skills, because they fit the circumstances just by the nature of the way this example is constructed.}