One of my party members wants a sword that can ignite (yes he is copying Vin Diesel). Instead of letting him just have it, I want to let him get the weapon enchanted, but the drawback is that he has to apply some kind of powder to the sword to light it on fire, hence rendering it an expensive weapon to use.

Is there some kind of item in the game which would be suitable to use for this? Otherwise I can just make one up.

Please clarify, you want

a) an enchanted weapon that can ignite?

b) an enchanted weapn that can only ignite with powder?

c) a mundane weapon that can ignite with powder?

Because something like the Flame Tongue Blade would be the Magic Item to go to. You can let your player create one, using the magic item creation rules (be careful with the cost).

I know in Neverwinter nights you could coat your sword in https://www.dndbeyond.com/equipment/alchemists-fire-flask  I'd probably let one flask add fire damage to a weapon for a few rounds

Alchemist's fire seems like a good choice, either that or you could make a charged magic item, that can be refilled. so like a vial of alchemist's fire would refill 3 charges.

If you want to go with a powder, gunpowder's the go to. If gunpowder's not in your game i don't know what to tell you.

You can also have flames coming off the blade with Continual Flame for just the cost of 50 GP of ruby dust.  It doesn't burn anything, but the only way to find out is to be smacked with the weapon... Now if he wants the full magical effect and flame damage, that's a different thing, but if he just wants it to LOOK cool, then that would be my go to... 

Ok, powder to ignite it... yeah, that's going to be something different entirely.  Sounds like homebrew territory.  And I would recommend they come up with it... and ask them detailed questions, "How do you think this should work?  What is a reasonable ingredient to use?  How much should it cost for the powder to ignite the blade?  Do you want it to do any additional damage?  If so, compare that to how much it costs for a spell of similar damage amounts and we'll find you an alchemist or herbalist to go to..." 

Let them do the work and if it seems reasonable, allow it.  If not... customize some more...

I could see that there might be some Resin Powder that would work.  They can be somewhat sticky and might burn for a few minutes.  Just need to come up with what is mixed with it and go from there....

Can powders really be sticky tho... i was envisioning some kind of canister in the blade with really small holes for a continuous burn.

Maybe they are only sticky when encountering metal?  So that putting it on your hand is fine, but when it touches metal it bursts into flame.  Maybe it is a specific metal... make him upgrade the weapon to Adamantium and that this one powder works only on that... who knows... but there is a lot of creation inspiration in following some of these ideas...

Quote from I_Love_Tarrasques >>

Alchemist's fire seems like a good choice

 with some guar thrown in to make it a paste/jel.

If it was magnetic? maybe, maybe iron pellets coated in gunpowder or another flammable substances, they'd fall off once they get burnt up since the heat would reverse the magnetism process, and it would look kind of cool as it would add a showering of sparks asthetic.

Quote from HeathSmith >>

Quote from I_Love_Tarrasques >>

Alchemist's fire seems like a good choice

 with some guar thrown in to make it a paste/jel.

 I mean i always thought of alchemist's fire as a tar like substance so it would already be sticky *shrug*

Quote from Noksa >>

 I mean i always thought of alchemist's fire as a tar like substance so it would already be sticky *shrug*

 i suppose, it does say sticky adhesive...maybe a special sheath that you fill with the goo so it ignites as soon as you pull it out, but the stuff inside the sheath remains stable.

kind of what i was thinking.

Couldn't he just be a Blood Hunter class, which literally has this ability?

I mean, it IS the class specifically made for Vin Diesel's Witch Hunter character, after all.

He gets the Witch Hunter character theme and flaming sword, and you don't have the headache of inventing stuff to recreate it. Seems like the simple solution.

Quote from Cyb3rM1nd >>

Couldn't he just be a Blood Hunter class, which literally has this ability?

I mean, it IS the class specifically made for Vin Diesel's Witch Hunter character, after all.

He gets the Witch Hunter character theme and flaming sword, and you don't have the headache of inventing stuff to recreate it. Seems like the simple solution.

 But what's the fun in that :D

Quote from I_Love_Tarrasques >>

Can powders really be sticky tho... i was envisioning some kind of canister in the blade with really small holes for a continuous burn.

 Powders can't stick to themselves or they aren't powders, they're a solid mass of gunk. They can stick to other objects, though I can't think of any real powders that are flammable and would stick particularly well to metal, unless you heat the blade by enough to melt whatever powder it is, in which case things like wax will work.

I would probably use the same material component as fireball (a tiny ball of bat guano and sulfur), which is itself a reference to gunpowder (bat guano was a standard source of nitrates for early gunpowder production, though it has been replaced by more convenient sources).

I know the discussion has already moved away from this, but I just wanna say that the powder should be whatever powder they throw into the fire at the start of each episode of Are You Afraid of the Dark

Quote from Pantagruel666 >>

Quote from I_Love_Tarrasques >>

Can powders really be sticky tho... i was envisioning some kind of canister in the blade with really small holes for a continuous burn.

 Powders can't stick to themselves or they aren't powders, they're a solid mass of gunk. They can stick to other objects, though I can't think of any real powders that are flammable and would stick particularly well to metal, unless you heat the blade by enough to melt whatever powder it is, in which case things like wax will work.

I would probably use the same material component as fireball (a tiny ball of bat guano and sulfur), which is itself a reference to gunpowder (bat guano was a standard source of nitrates for early gunpowder production, though it has been replaced by more convenient sources).

 Like i said if it was magnetic that might work, it would have to be some type of flammable iron compound (which should be doable)

If he used gunpowder that would definitely work, idk if it's allowed in that campaign but it is official content.