Yes, the title may sound insane, but it is very possible, fairly easy even, with two relatively cheap and common magic items:  dust of dryness, and a custom made unbreakable arrow.

Step 1:

Sprinkle some dust of dryness in a lake or other body of water. This creates a water marble which holds an 15x15 foot cube (3375 cubic feet) of water. This is the “warhead” of your sun arrow.

Step 2:

make or order a custom hollow arrow that will fit the orb inside. The arrowhead must be put on after you (carefully) put the orb inside.

Step 3:

Use an artificer infusion, or a few days of magic work to turn this into the common magic item: unbreakable arrow.  The wording states quite plainly that it simply CANNOT break outside of an antimagic field.

When the water marble is broken, it releases the 15x15 foot cube of water (3375 cubic feet) into the space around it.  The unbreakable arrow acts as a very nice container for this huge amount of water, creating compressed water.  Normally, water is essentially incompressible. However, since it has nowhere else to go, it stays inside the arrow.  Since compression creates heat, the water will become superheated plasma, and 100k pounds of it at that.

there is no ruling on how much damage this would do, but it’s safe to say a multimillion (maybe billion) degree compressed plasma arrow weighing more than most houses would hurt a bit.  The best part is, it automatically triggers on a hit, since the water marble hits the front of the arrow, shattering instantly.

(disclaimer: though this would probably work in a few campaigns, please don't try it for the sake of your DM. this is a purely hypothetical weapon, for the fun of thinking about high amounts damage we will never do)

*slides on DM hat*

At the moment you put that marble into the arrow, the marble is also unbreakable, because at that moment the marble is part of the arrow (at leas,t in terms of the game’s underlying mechanics that would treat it as a single object for purposes of damage). THerefore, you don’t get any further compression (since you are using real world physics, not D&D physics to create a plasma that would have been created by the compression fo the water by the dust in the first place), and while your mass estimate seems fairly reasonable as I am so not going to break out my calculator and do the math, it also presumes that D&D operates on “real world physics”, which it does not do.

*takes off DM hat — which is actually the hood of a black robe, not a hat, but meh*

heh. Cool thought. Id probably allow it once. FOr the sheer fun — but not at a stellar scale, because that would ultimately wipe out the entire party (based on range, gravity, and what happens when that much plasma decompresses), but still to a degree of decent potency.

if you really want to annoy your DM, in a space without much air movement, release a bag of flour and have them calculate a dust explosion.

I'd rule that the arrowhead being removable in your design, means that when the bead broke, the arrowhead would detach, and it would be flung forward as the rest of the arrow is flung backward at the release of water.  I'd probably make the arrow deal critical hit damage in addition to the effect of the Decanter of Endless Water's Geyser, while a random creature within 60ft of the target in the direction towards the shooter has to make a Dexterity saving throw or take the arrow's normal damage, as the back of the arrow flies at the creature at high speed. 

The Plasma Arrow idea is a lot like the peasant railgun, in that it's a fun idea, but doesn't actually work mechanically.

(forgot about quoting, this is replying to fayettegamer’s first post)

Good point, one way to counteract that would be sovereign glue, it would only take a drop or 2 to get the job done, but it would be prohibitively hard to get any.

Also, this was originally just meant to be a pressure washer type thing.  My original plan when coming up with this was if the arrowhead had a tiny hole in the front, it would cause a water jet to do tons of damage to the creature (and creatures directly behind it) but then I realized if it was closed it would be a much more interesting weapon.

(forgot to use the quote feature, this is replying to AEDorsay’s post)

If you  made it with a minuscule hole in the front of the arrowhead, you could put the dust inside, then place it underwater. After all, it never said it had to have a super open contact to the 15 foot cube of water. (The hole would also allow pressurized plasma to escape from somewhere easier than microscopic cracks and indestructible wood grain)

This is a very good point though, and I would use some sort of logic like this to rule against this OP artificer nuke of an idea.

It’s a funny idea, but I’d just say no. You can either do something with magic or physics. Combining them doesn’t work (personal rule of thumb).

Quote from ShhinySilver >>

(forgot to use the quote feature, this is replying to AEDorsay’s post)

If you  made it with a minuscule hole in the front of the arrowhead, you could put the dust inside, then place it underwater. After all, it never said it had to have a super open contact to the 15 foot cube of water. (The hole would also allow pressurized plasma to escape from somewhere easier than microscopic cracks and indestructible wood grain)

This is a very good point though, and I would use some sort of logic like this to rule against this OP artificer nuke of an idea.

 

 Yeah…. no.

“You can use an action to sprinkle a pinch of it over water.“ has to be sprinkled over the water, not the water added to it. Does t say super open contact, but it does say it has to be sprinkled over the water.

also, again, don’t forget that this is D&D physics, because if it was real world physics that marble would weigh the same as that much water and it would be compressed into a plasma state (not to mention undergo a phase shift, holy cow, it is a thermonuclear weapon, the old “hard water”)…

… but wait! There’s more!

it doesn’t do any damage when it expands, so there is t a lot of force or kinetic energy in the expansion. We know this because if it did, there would be damage listed for the water. There isn’t, only the powder against a water elemental.

so there would be no explosion! Just a bunch of water flooding the area!

this is why I giggle when players bring “real physics” into a game where falling 200 feet is survivable enough you can get up and keep fighting or running a marathon. (That’s 20d6, so minimum is 20 points. Max might be killer, but minimum can happen, lol).

d&d *does have physics*, mind you. They just aren’t real world physics, lol.

EDIT:

so, as a result of some homebrew I am working on, and to drive home the whole physics thing, lol, a Speed of 1 is equal to 600 feet in an hour, or 10 feet in six seconds.

not a surprise.

but a speed of 60 mph — the traditional short burst of a cheetah — is 528 by that same comparison (which really drives home the blur factor, lol).

a speed of 3 mph (4.8km/h) is the typical average walking speed for an adult, with a short burst of 4 mph. That translates to 26.4 to 35.2 as a speed. 
RAW, however, humans have a speed of 30. So PCs are rushing, lol. as in, really moving quickly. Fast march kinda stuff.

real world is so not the same, lol.

(this is my punishment for needing to fix vehicle mechanics)

Compression generates heat. It is not, however, being compressed. The compression happens when you sprinkle the dust. If you want real world physics, that should generate a lot of heat. However, once the marble breaks, it's being kept from expanding. Zero energy is released. 

Wonderful for firefighting, though.

Quote from Acromos >>

Compression generates heat. It is not, however, being compressed. The compression happens when you sprinkle the dust. If you want real world physics, that should generate a lot of heat. However, once the marble breaks, it's being kept from expanding. Zero energy is released. 

Wonderful for firefighting, though.

 I would have to argue against this.  Have you ever tried to compress water? The most you are ever going to get is so minuscule you shouldn’t bother measuring it.  It takes full on labs to compress it by just a few percent (or compressed water pipes) what we are talking about here is compressing a cube of water weighing as much as a house into an area hundreds of thousands, or millions of times smaller.  The force the water would exert would compress itself into superheated plasma almost instantly.  I even had to take into account nuclear fusion for this, I can’t remember if it was hot enough for it thoug.

Quote from ShhinySilver >>

 I would have to argue against this.  Have you ever tried to compress water? The most you are ever going to get is so minuscule you shouldn’t bother measuring it.  It takes full on labs to compress it by just a few percent (or compressed water pipes) what we are talking about here is compressing a cube of water weighing as much as a house into an area hundreds of thousands, or millions of times smaller.  The force the water would exert would compress itself into superheated plasma almost instantly.  I even had to take into account nuclear fusion for this, I can’t remember if it was hot enough for it thoug.

 You are not compressing the water. Rather, it stays compressed. 

The compression - and hence the theoretical release of energy - takes place when the spell is cast. When the spell ends, nothing happens. The water remains compressed, because it cannot do otherwise. No plasma. When it's released from the arrow, you get precisely what the spell says: 15'x15' of water. No explosive release or anything dramatic of any sort. 

Although 15'x15' of water can have it's uses. Like, if you want to clean a teenager's room, or create a humorous trap above someone's door. 

Quote from ShhinySilver >>

Quote from Acromos >>

Compression generates heat. It is not, however, being compressed. The compression happens when you sprinkle the dust. If you want real world physics, that should generate a lot of heat. However, once the marble breaks, it's being kept from expanding. Zero energy is released. 

Wonderful for firefighting, though.

 I would have to argue against this.  Have you ever tried to compress water? The most you are ever going to get is so minuscule you shouldn’t bother measuring it.  It takes full on labs to compress it by just a few percent (or compressed water pipes) what we are talking about here is compressing a cube of water weighing as much as a house into an area hundreds of thousands, or millions of times smaller.  The force the water would exert would compress itself into superheated plasma almost instantly.  I even had to take into account nuclear fusion for this, I can’t remember if it was hot enough for it thoug.

 
so, um, yes. I have compressed water many times. Hundreds of them. 

In game. Where the physics are different from the real world, lol. How? Well…

Magic!

That said, you stepped a bit too far into fusion principles, because this doesn’t include fusion — plasma theory should be enough. The moment you shift to fusion principles, you have random collisions and those would occur during the compression phase (with concomitant results of an explosive nature during that phase, and that would kill everyone using the darn powder), since the isotope that would generated through compression is not going to work well with impurities int he water and would split the resulting molecules, and that in turn would result in fissionable separation and collision (chaos, baby) that would trigger at the least a hydrogen explosion given more power by the oxygenation.

Thankfully, real world physics don’t apply. I mean, the physically unconstrained properties in just getting the marble is tremendously dangerous, and the necessity of magic as your containment method is pretty much a promise that from the get go you cannot apply normal physics to the game — because magic ignores physics.

Quote from Acromos >>

Quote from ShhinySilver >>

 I would have to argue against this.  Have you ever tried to compress water? The most you are ever going to get is so minuscule you shouldn’t bother measuring it.  It takes full on labs to compress it by just a few percent (or compressed water pipes) what we are talking about here is compressing a cube of water weighing as much as a house into an area hundreds of thousands, or millions of times smaller.  The force the water would exert would compress itself into superheated plasma almost instantly.  I even had to take into account nuclear fusion for this, I can’t remember if it was hot enough for it thoug.

 You are not compressing the water. Rather, it stays compressed. 

The compression - and hence the theoretical release of energy - takes place when the spell is cast. When the spell ends, nothing happens. The water remains compressed, because it cannot do otherwise. No plasma. When it's released from the arrow, you get precisely what the spell says: 15'x15' of water. No explosive release or anything dramatic of any sort. 

Although 15'x15' of water can have it's uses. Like, if you want to clean a teenager's room, or create a humorous trap above someone's door. 

 yes, it does stay compressed to a degree, but that much water in such a tight space wouldn't want to just... stay there normally, and the water would be exerting incredible forces on the container and itself, therefore compressing it.  the dusts description specifically states the water comes directly out of the orb into the space around it. since that may take a split second, the water rushing out of the orb would begin to compress from itself. in addition, compressed water has some real power behind it, in fact, waterjets, which only compress water to about 11% smaller can cut through steel, now if you take into account that this arrow would be compressing it to nearly a MILLION times smaller, that would create some insane heat and forces, therefore creating compressed plasma.

edit: not to mention the weight, since the arrow would suddenly be holding 100 tons of water when it hits someone, all that weight in such a small area would cause it to fall through them and the floor like the worlds heaviest sword.  that would essentially turn it into a rod even more immovable than an immovable rod (except for gravity) to slice right through them.

Quote from ShhinySilver >>

 yes, it does stay compressed to a degree, but that much water in such a tight space wouldn't want to just... stay there normally, and the water would be exerting incredible forces on the container and itself, therefore compressing it.  the dusts description specifically states the water comes directly out of the orb into the space around it. since that may take a split second, the water rushing out of the orb would begin to compress from itself. in addition, compressed water has some real power behind it, in fact, waterjets, which only compress water to about 11% smaller can cut through steel, now if you take into account that this arrow would be compressing it to nearly a MILLION times smaller, that would create some insane heat and forces, therefore creating compressed plasma.

edit: not to mention the weight, since the arrow would suddenly be holding 100 tons of water when it hits someone, all that weight in such a small area would cause it to fall through them and the floor like the worlds heaviest sword.  that would essentially turn it into a rod even more immovable than an immovable rod (except for gravity) to slice right through them.

 It is the action of compressing it that heats it. That action takes place when the spell is cast. Not when the sphere is broken. So when the sphere is broken, the water is simply contained in some state - super compressed, but stable since it literally can't go anywhere - until the arrow breaks. Then, the water will expand rapidly to it's normal volume. 

Since we're outside the realm of both science and reason, we cannot really say anything meaningful about what happens - but there's really no reason to expect a particularly powerful effect, because the item description doesn't in at any such option in any way, shape or form. 

The item doesn't do anything it doesn't state clearly on the tin. But that's an aside to the fact that your physics are plain wrong. 

Quote from Acromos >>

Quote from ShhinySilver >>

 yes, it does stay compressed to a degree, but that much water in such a tight space wouldn't want to just... stay there normally, and the water would be exerting incredible forces on the container and itself, therefore compressing it.  the dusts description specifically states the water comes directly out of the orb into the space around it. since that may take a split second, the water rushing out of the orb would begin to compress from itself. in addition, compressed water has some real power behind it, in fact, waterjets, which only compress water to about 11% smaller can cut through steel, now if you take into account that this arrow would be compressing it to nearly a MILLION times smaller, that would create some insane heat and forces, therefore creating compressed plasma.

edit: not to mention the weight, since the arrow would suddenly be holding 100 tons of water when it hits someone, all that weight in such a small area would cause it to fall through them and the floor like the worlds heaviest sword.  that would essentially turn it into a rod even more immovable than an immovable rod (except for gravity) to slice right through them.

 It is the action of compressing it that heats it. That action takes place when the spell is cast. Not when the sphere is broken. So when the sphere is broken, the water is simply contained in some state - super compressed, but stable since it literally can't go anywhere - until the arrow breaks. Then, the water will expand rapidly to it's normal volume. 

Since we're outside the realm of both science and reason, we cannot really say anything meaningful about what happens - but there's really no reason to expect a particularly powerful effect, because the item description doesn't in at any such option in any way, shape or form. 

The item doesn't do anything it doesn't state clearly on the tin. But that's an aside to the fact that your physics are plain wrong. 

I am quite sure the physics aren’t the problem, think of it this way.  If you had an unbreakable closed pipe with  a one sided portal to a lab of highly compressed water on one end, what do you think would happen?  it would compress, because it has nowhere else to go, and the portal is one - way, and therefore compress. that is pretty much 1 - 1 what happens in the arrow.

the one thing that would be a problem to this is a reasonable DM that doesn’t want nukes in their campaign.  (I highly doubt I would let this nuke to the degree it would in real life, but I would probably let it explode with decent power for the effort the players put in)

Quote from ShhinySilver >>t that your physics are plain wrong. 

I am quite sure the physics aren’t the problem, think of it this way.  If you had an unbreakable closed pipe with  a one sided portal to a lab of highly compressed water on one end, what do you think would happen?  it would compress, because it has nowhere else to go, and the portal is one - way, and therefore compress. that is pretty much 1 - 1 what happens in the arrow.

the one thing that would be a problem to this is a reasonable DM that doesn’t want nukes in their campaign.  (I highly doubt I would let this nuke to the degree it would in real life, but I would probably let it explode with decent power for the effort the players put in)

 No. It's already compressed. I simply don't know how else to explain it. It is compressed. It is not being compressed. It is the action of compressing it that creates the heat. Not the state of being compressed. Quite literally, nothing will happen. Like, at all. 

If the arrow is unbreakable, how would the plasma deal any damage at all? The plasma is trapped within the arrow maybe dealing 2d8 fire damage for heating up the arrow head, but since the plasma wouldn't be released the unbreakability of the arrow would prevent the plasma from contacting the target so would not deal serious damage.

The above poster has a good point too. The dust of dryness has compressed the water molecules far more than the unbreakable arrow would do, so the water being released would decompress it and actually absorb heat not release it.

From someone who loves Physics and D&D:

1: If the arros is unbreakable, how does the marble inside break?

2: If the plasma appears inside the unbreakable arrow, how does it damage things outside?

3: It being magic, why would the water emerge from the broken marble and not magically appear in a cube around it?

4: Agreeing with Acromos, the issue you have is that the water you are releasing is already compressed, and would like to expand. It cannot expand, so it remains compressed - it does not become compressed.

The reason for this (physics time!) is that the specific heat capacity of a material changes as it is compressed. The amount of energy per unit volume increases, because the volume decreases, which means that the temperature increases. As such, you would need to take the water in an already expanded state, and then compress it into a smaller volume, in order to increase the temperature in that volume of water.

Technically speaking, if we apply physics to magic, when you use the dust, the marble would become incredibly hot. Similarly, once it has cooled, if you break it, the water will decompress. The volume will increase, and the energy per unit volume will drop - you can't create energy - and the water will cool, probably well below freezing point. The speed would probably turn it into a freezing fog.

As a DM, I would probably say that the unbreakable arrow would stop the marble breaking. If you worked around that, I would say that the arrow has a chance of breaking the marble on launch, and that it would "spawn" water in a cube around it - though I would absolutely allow that water to have the same speed as the marble had. This would likely turn out to be a Tidal Wave Arrow, which is itself an extremely cool thing.

I do love stuff like this!

The big problem is that you assembled the arrow, including the marble BEFORE converting it to an unbreakable arrow. Once that happens the entire arrow is unbreakable - including the connection between the shaft and the head AND the marble in the head. So unless your target is inside an anti magic zone nothing happens. 
Inside the anti magic zone it might get more interesting but probably not too much as the shaft, head and marble would have to be touching solidly so there is no “wriggle room” or the marble’s motions inside the arrow during flight would distort the path making it inaccurate so that it is most likely to miss, fly off and land somewhere still without breaking. Even if it hits the shaft and head have to have a solid contact keeping the shaft from moving forward or it would smash the marble as the bowstring drives it forward blowing the wielder up and not the target. Any chamber the marble is in that is large enough for the marble’s independant inertia in it to break the marble on impact is also Large enough for the inertia to break the marble on launch ( unless launch occurs outside the anti magic area where the arrow ( and marble) are unbreakable. So at best this is usable only when firing at a foe inside an anti magic area when you are outside it - pretty restricted usage. Even inside the area the effect is not what you might think. Because the water is expanding fairly instantaneously it is going to do basically 2 types of damage: first cold damage as expanding materials cool not heat. Since the arrow is already room temperature it’s going to create a 15’ radius area of snow  as the water expands. The temperature will drop significantly but not to lower than that of a cone of cold or white dragon’s breath so 8-12 d8 cold damage in the 15’ radius with a Dex save to all except the target in the area. Second, the snow is blasting out at high speed to fill the area knocking folks off their feet sending them flying/falling so 2 D6 bludgeoning damage ( like a 10 to 20’ fall) plus a second Dex save or gain the prone condition.