Cloudkill (spell)

Cloudkill is a spell that conjures a roiling mass of sickly yellow-green vapours, forming a toxic cloud that spreads outward from the caster. The vapours are lethal to creatures with fewer than 4+1 hit dice, while those with 4+1 to 5+1 hit dice must save versus poison with a -4 penalty. Creatures of up to 6 hit dice also must make save, but without penalty. Once created, the cloud moves away from it's point of origin at a steady pace of 10 ft. per round, creating a wider and wider circle, remaining close to the ground as it rolls outward. The height of the cloud is 20 ft.

Cloudkill

Range	10 ft. per level
Duration	1 round per level
Area of Effect	15 ft. diameter cloud; see text
Casting Time	3 rounds
Saving Throw	none
Level	mage (5th)

The caster and others are not immune to the effects of the spell and must likewise give ground before it. Once put in place, the expanding cloud is no longer magical; it cannot be dispelled or whisked out of existence by the caster; the gas remains until the duration is completed, whereupon it finally thins and dissipates to where it no longer requires a saving throw. This is true even if the gas is contained, for it's lethality effectively has a chemical half-life that ends when the duration is complete. Thus the gas cannot be bottled and cannot be seen as an ongoing hazard in a sealed off room.

Poisonousness

While the spell is certainly lethal to those with fewer than 4+1 hit dice, all other creatures might survive even if they fail their saving throw. Instead of instant death, exposure to the toxic vapours inflicts 5d6 +30 points of poison damage. This ensures that even powerful creatures suffer serious harm, but outright death is no longer an immediate consequence of failing the save. Those making their save are assumed to have effectively held their breath and moved free of the cloud's effects, but only for that round — the same save must be made every round that the character is within the gas. Those that have fallen and been knocked unconscious, who are almost certain to be inside the cloud the following round, cannot make save again and thus must suffer the full effects of the toxicity each round.

Wind

The wind force within the area of effect influences the cloud in numerous ways. While a calm or light air have no effect, a light breeze compels the cloud to expand only in the direction leeward of it — though the speed of expansion is not change. A gentle or moderate breeze will double the speed of the cloud in the direction of the wind, while the latter will erode the cloud by ten feet towards windward. A fresh breeze will begin to cut the cloud to ribbons, making it harmless in 2-5 rounds, such that while the cloud expands leeward, the actual toxic effect expands at the normal 10-foot pace — the tailings beyond that are too weak to require a saving throw or kill even lesser creatures. A strong breeze breaks up the cloud within 1-3 rounds; any greater wind force breaks up the cloud as soon as it appears.

A gust of wind destroys as much of the spell as the blast of air can touch, though once the wind has ceased — so long as there is any cloud left — the cloud will continue to expand back again into areas that were cleared away. A zephyr can reduce the cloud within its area of effect, but once the zephyr passes (likewise 10 ft. per level) the cloudkill reasserts itself unaffected.

Vegetation

Thick, dense undergrowth, tangled roots or a heavy forest canopy, interferes with the cloud's ability to spread. As the vapours push through, they are gradually absorbed and dissipated, breaking apart the concentrated toxicity. The cloud disperses entirely after travelling two rounds' worth of distance through thick vegetation, meaning a sufficiently dense forest can neutralise the spell before it reaches its intended targets. Lighter vegetation, such as scattered trees, shrubs or tall grasses, has no particular influence over the cloud's spread.