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At level 12, alchemists can make poison bomb

Poison Bomb

Benefit: The effects of the smoke created by an alchemist’s bomb duplicates the effects of cloudkill instead of fog cloud, filling an area equal to twice the bomb’s splash radius for 1 round per level.

It duplicates the effects of cloudkill :

Cloudkill

Effect : cloud spreads in 20-ft. radius, 20 ft. high

This spell generates a bank of fog, similar to a fog cloud, except that its vapors are yellowish green and poisonous. These vapors automatically kill any living creature with 3 or fewer HD (no save).

[..]

Unlike a fog cloud, the cloudkill moves away from you at 10 feet per round, rolling along the surface of the ground.

Figure out the cloud’s new spread each round based on its new point of origin, which is 10 feet farther away from the point of origin where you cast the spell.

Because the vapors are heavier than air, they sink to the lowest level of the land, even pouring down den or sinkhole openings. It cannot penetrate liquids, nor can it be cast underwater.

When they are created by wizard, cloudkill moves away from the wizard. What happened when they are generated by an alchemist's bomb ?

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There are at least two possible readings of the alchemist discovery poison bomb

The discovery says, "The effects of the smoke created by an alchemist’s bomb duplicates the effects of cloudkill instead of fog cloud [that's created by the prerequisite discovery smoke bomb], filling an area equal to twice the bomb’s splash radius for 1 round per level." However, what that means isn't entirely clear.

Maybe it's precisely like cloudkill

This reading emphasizes the bomb's effect's similarity to the spell cloudkill. Using such a reading, the cloudkill effect behaves as any other cloudkill effect, so it "moves away from [the affected area and, probably, the alchemist] at 10 feet per round, rolling along the surface of the ground"; the GM must "[f]igure out the cloud’s new spread each round based on its new point of origin, which is 10 feet farther away from the [original] point of origin where you cast the spell"; and the effect "sink[s] to the lowest level of the land, even pouring down den or sinkhole openings[, but i]t cannot penetrate liquids, nor can it be cast underwater."

This GM thinks—and this fine answer ultimately agrees—that this reading likely overextends the mandate of the discovery poison bomb. That is, this isn't a particularly superpowerful reading or anything—the level 12 alchemist, at this point, has devoted 1/3 of his discoveries to smoke bombs that in a much smaller area and for a much shorter duration create an effect that a wizard's been creating for the last three levels—, but, simply, one that, in this GM's opinion, goes beyond the text… and, not inconsequentially, is also enormously complicated, making this GM dread the alchemist ever hucking even a lone poison bomb.

…Or only the smoke has an effect like cloudkill

This reading emphasizes the opening description of the discovery poison bomb: "The effects of the smoke created by an alchemist’s bomb duplicates the effects of cloudkill." So the smoke created by the discovery poison bomb is unlike fog cloud's effect that

obscures all sight, including darkvision, beyond 5 feet. A creature within 5 feet has concealment (attacks have a 20% miss chance). Creatures farther away have total concealment (50% miss chance, and the attacker can’t use sight to locate the target).

Instead, the effects of smoke created by the discovery poison bomb are like the effects of the smoke created by the cloudkill spell, which are

similar to a fog cloud, except that its vapors are yellowish green and poisonous [so still having the sight-blocking effects above]. [Plus t]hese vapors automatically kill any living creature with 3 or fewer HD (no save). A living creature with 4 to 6 HD is slain unless it succeeds on a Fortitude save (in which case it takes 1d4 points of Constitution damage on your turn each round while in the cloud).

A living creature with 6 or more HD takes 1d4 points of Constitution damage on your turn each round while in the cloud (a successful Fortitude save halves this damage). Holding one’s breath doesn’t help, but creatures immune to poison are unaffected by the spell.

Thus, under this reading, the effects of the smoke—the damage it deals and its appearance—are like cloudkill, but the GM has ruled that movement of the smoke away from the creator and down into pits or warrens or whatever is not an effect of the smoke so the poison bomb smoke remains stationary as per the fog cloud spell.

While this reading is a little strained, it's far more playable than the first alternative. Considering the sheer number of poison bombs an alchemist at this level can drop in only a few rounds, keeping track of the number of drifting cloudkill effects—even for a few rounds and especially in a highly-populated area!—is a GM nightmare, making this, too, a reasonable ruling.


Note: While the question's title asks How does a cloudkill generated by the alchemist discovery poison bomb naturally disperse? that answer is as per the fog cloud spell: "A moderate wind (11+ mph) disperses the [effect] in 4 rounds; a strong wind (21+ mph) disperses the [effect] in 1 round," or, alternatively, as per the discovery smoke bomb, the effect dissipates naturally in 1 round/alchemist level. However, I think the actual question is Does a poison bomb's smoke move like the smoke from the cloudkill spell? but the answer to the title's question is here for completeness.

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  • \$\begingroup\$ I was surprised at a number of threads discussing this discovery: two. None of which received much attention from the community. \$\endgroup\$ – ShadowKras Jul 12 '17 at 17:14
  • \$\begingroup\$ @ShadowKras Me, too! The lack of threads discussing it leads me to believe that tables either naturally agree on what happens or just don't care what happens because, screw it, do what you want: you're a level 12 alchemist and you're way too complicated anyway. \$\endgroup\$ – Hey I Can Chan Jul 12 '17 at 17:18
  • \$\begingroup\$ keeping track of the number of drifting cloudkill effects—even for a few rounds and especially in a highly-populated area! moving darkness areas are bad enough... \$\endgroup\$ – ShadowKras Jul 12 '17 at 17:33
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    \$\begingroup\$ @ShadowKras Yeah, especially if the alchemist urges the party to wear goz masks or something. I can imagine the GM just throwing up his hands and saying, "Okay, you win. Now what?" rather than playing that all out. \$\endgroup\$ – Hey I Can Chan Jul 12 '17 at 17:40
  • \$\begingroup\$ especially if the alchemist don't throw all the bombs in the same directions and/or moves between different throws. \$\endgroup\$ – Anne Aunyme Jul 13 '17 at 8:39
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You did a good job at piecing together all that mess. From a rules as written reading of all that, the cloud should move alway from the point of impact based on the position of the alchemist when the cloud is created, at speed of 10 feet per round.

You will notice that Poison Bombs requires the Smoke Bomb discovery, and it even mentions it in the ability's text.

Poison Bomb

Prerequisite: Alchemist 12, smoke bomb discovery

Benefit: The effects of the smoke created by an alchemist’s bomb duplicates the effects of cloudkill instead of fog cloud, filling an area equal to twice the bomb’s splash radius for 1 round per level.

Here, the bolded part is talking about bombs created using Smoke Bomb discovery, which works as Fog Cloud are dispersed by winds:

A moderate wind (11+ mph) disperses the fog in 4 rounds; a strong wind (21+ mph) disperses the fog in 1 round.

There is nothing on Cloudkill that changes this. It simply adds another movement to the effect.

That means that the effect of Poison Bombs will both move each round and be dispersed by strong winds.

However, I do not believe this is the intent of the developer who wrote this. As the text is over simplistic for a spell that behaves completely different from the other it replaces (one moves, the other doesn't). What was supposed to simply be a poisonous special effect on top of a smoke bomb, turned out to be something really different from the original effect of the smoke bomb. As such, I wouldn't be surprised if this receives an errata or your GM changes it's behavior to not move.

This also leads to strange behaviors, like: Would it make more sense to throw the bomb before a group of enemies, so the cloud actually affects them at least twice before moving away from their area and even possibly affect a larger number of enemies, or to throw on top of them and actually cause the bomb's damage (FAQ reference) in addition to the cloud's effect?

The bomb does have a splash radius of 20-feet, meaning that in two turns, those affected by the far side of the initial area will be safe from it, and in one round, those on the closest side of this area will no longer be affected.

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