How can I most clearly write a homebrew item that affects the ground below its radius after the initial explosion it creates?

Question

This item, in essence, releases a burning chemical that deals fire damage to and ignites creatures and objects

1. within its initial sphere radius
2. everything beneath the explosion of the bomb

It then ignites the ground beneath its explosion, in the same area as the explosion. It's basically an alchemical incendiary. The area of effect originates from a point you choose/throw the object to within 60 feet of you. It should work like a spell effect in that regard. I'm thinking the initial damage deals 3d6 fire damage, and another 3d6 at the start of each ignited creature's turns until they take an action to put it out. Same damage—or perhaps less—when a creature enters a burning area.

I just have no clue how to write it properly based on how I want it to work.

For example:

Paladin Pentagon is in the bomb's blast radius. So is Square Druid in her wild shape. Star aaracokra ranger and Hexgoblin are not in the initial blast radius. Pentagon and Square are set alight because they're in the initial explosion. Then the stuff falls in the radius of the bomb to create a burning circle on the ground. Square is not in the ground circle, but she's still on fire because she was caught in the intial blast. Hexgoblin wasn't in the initial blast, but he's still set on fire because he was beneath it. Pentagon Pally would be set alight either way. Star isn't set on fire at all.

I really have no clue how to word this area of effect, or at least something similar. So far I've considered:

• a cone that must be oriented downward to create the expansion/explosion effect and the circle on the ground. Problem: can spells and effects written like spells force the orientation of a spell effect? I don't think they can.
• a sphere and a cylinder straight down, both of which use the origin point of wherever the bomb exploded. A creature can only take the damage once per turn. I think this one works, but my gut says something about rotating columns along axes that change the direction from "straight down" could be an issue.
• a sphere of effect, and then affects a circle of the same radius on the ground beneath it.
• Just be done with it and do a cylinder. Problem is, if the cylinder is thrown high enough, it won't hit the ground at all, and for something that's functionally napalm that doesn't make a whole lot of sense to me.

I hate to ask stack exchange to write my stuff content for me, but I'm at a bit of a loss here and figured folks would appreciate a challenge.

Answer 1

So, say the spell creates an explosion/area-of-effect that is a sphere with radius n. However, this explosion creates "shrapnel" of a sort which falls to the ground from the explosion, presumably in the direction of gravity, and sets everything that it touches on fire, including the ground. (This is my reading of your effect, please let me know if it is incorrect.)

Mathematically, what you are doing is projecting the sphere onto the ground. Think of the ground as your regular 2D (x,y) coordinate plane. If you make the center of the spherical explosion the point (h,j,k), then the center of the projection circle will be the point in 3D space (h,j,0). I.e. the center of this circle will be straight down on the ground from the point of origin of the sphere.

So, basically, your spell will ignite everything caught between this sphere (the original area of effect), and this circle (the projection of the sphere) on the ground. The shape this will create is going to look like a right circular cylinder (the can-shaped cylinder we all know and love) with radius n and height k, that is topped by the upper hemisphere of the original explosion. This cylinder will include the bottom half of the original sphere, because its height goes up to the center of the sphere. And since they have the same radius n, the upper hemisphere will be sitting perfectly on top of this cylinder.

All of that taken into account, I think the proper way to write this item effect would be:

This item, when thrown, explodes in mid-air (you will need to specify how to tell where the item explodes at, it was unclear), exploding and damaging every creature within a sphere of radius n from the item for x (damage was unspecified) type of damage. Every flammable creature or item within this sphere is also set on fire (You would need to specify what effect being on fire has, e.g. like the spell Immolation does). Additionally, every flammable creature or item in a cylinder of radius n extending from the point of origin of the sphere down to the ground (in the direction of gravity), is set on fire as well.

Answer 2

For my proposed wording, I've simply focused on boiling the item down to its essentials, dialing the numbers (damage, duration, etc.) back to minimal values; feel free to tweak based on how powerful you actually want this item to be.

Jar of Napalm

Minor, Uncommon
1lbs.

This magically sealed jar contains two substances that, when combined, produce a sustained explosion. These two substances are separated by a thin magical barrier that can be removed by twisting the lid of the jar. The barrier resists concussive force or shaking, but if the jar is ruptured or destroyed, the barrier dissipates.

As an action, you may twist the lid of the jar and then throw it to a point of your choosing within 30 feet. When the jar reaches that point, it creates a fireball in a 10 foot radius. Creatures caught within this fireball must make a Dexterity Saving Throw (DC 11), taking 1d6 Fire Damage on a failed save, or half on a successful save.

After this explosion, the ground beneath the jar is ignited in a radius equal to the explosion. Any creature that starts its turn within this circle or enters it for the first time during a turn must make a Dexterity Saving Throw (DC 11) and take 1d6 Fire Damage on a failed save, or half on a successful save.

Whenever a creature fails a saving throw to avoid taking damage from this effect, or a flammable object takes damage from this effect, they become ignited, receiving 1d6 Fire Damage at the start of each of their turns. Any ignited creature may use an Action to make a Dexterity Check (DC 10) and extinguish themselves on a successful check.

All flames produced by this item are extinguished after one minute, though flammable objects set ablaze might continue to burn.

If the substances in the jar make contact without the jar being thrown, they ignite and explode at the end of the same turn in which they made contact, producing the effects above.

Below are some notes on how I've worded this.

Multiple Effects

There are, by my understanding of how you want the item to work, several key parts to making the item work:

• A thrown item, or otherwise an effect targetable as part of the use of the item
• An initial explosion
• A lingering AOE hazard
• Ignition of characters and objects affected by the explosion or hazard

I've tried to incorporate each of these concepts as elegantly as possible, emphasizing the plain English description of these effects.

The Physics

To be honest, I think you're overthinking your concerns about the physics of this object. Certainly, if there's a frame-of-reference shift where Gravity no longer pulls downwards, the DM will need to work out how to handle that. My advice is to just do as I did, specifying the ground beneath the explosion as where the hazard is formed. This might lead to strange situations where, RAW, this item doesn't behave as expected, but as a homebrew item, we should trust that DMs will make the right call when determining how this item should behave.

Rarity/Power Level

Because of how involved the item is, it's probably ideal if the item have a relatively high rarity. I rated this version of the item as Uncommon to put it on par with its relative power level, but because the item is relatively complex in its mechanics, it's probably better for its rarity to be Rare (and have its power bumped up a bit to compensate) just to reflect the mechanics not having to be adjudicated as frequently. It's up to you though, and "Rarer == More Complex" is only a guideline that I follow, it's not a guideline that 5e dictates or follows in its own design.

Answer 3

Seems you can just use the effects of a couple of spells. This sounds like one time use fireball:

A bright streak flashes from your pointing finger to a point you choose within range and then blossoms with a low roar into an explosion of flame. Each creature in a 20-foot-radius sphere centered on that point must make a Dexterity saving throw. A target takes 8d6 fire damage on a failed save, or half as much damage on a successful one.

The fire spreads around corners. It ignites flammable objects in the area that aren't being worn or carried.

...with the fire effects of create bonfire:

You create a bonfire on ground that you can see within range. Until the spell ends, the magic bonfire fills a 5-foot cube. Any creature in the bonfire’s space when you cast the spell must succeed on a Dexterity saving throw or take 1d8 fire damage. A creature must also make the saving throw when it moves into the bonfire’s space for the first time on a turn or ends its turn there.

Answer 4

Describe it in physical terms, like you do here.

This bomb disperses a cloud of Bad Stuff (I'd call it "burning pitch" or "brimstone" or possibly "embers") in a sphere of radius X, which then falls to the ground and continues burning for Y rounds. Any creature or combustible object that contacts the Bad Stuff gets set on fire.

(Note that this avoids the "taking damage twice" issue by having the Bad Stuff be a single effect that moves. If you want to make this absolutely clear, append "if it isn't on fire already.")

Don't overthink it. You don't have to shoehorn this into one of the standard spell effect geometries. It's a sphere that moves, so describe it as a sphere, and then describe how it moves. See cloudkill for a PHB example like this.

One issue with describing it this way is that anything that depends specifically on an "area of effect" (the Sculpt Spell metamagic comes to mind) needs to know whether the area includes everything touched by the cloud, or just the original sphere. This may not be a problem in this case because it's not a spell, but if you were writing it as a spell, it would be wise to specify one or the other.