|This spell conjures forth a great cloud of lethal gasses.|
This spell creates 9 long lasting poison clouds, they will stretch out in a long line if cast in a corridor, otherwise it will be be 3x3 square. Note that this spell uses smite targeting, and will prompt if the cloud would hit you.
It's not that hard to cast. Damage per turn (subjective - slugs won't absorb much poison) is low, but damage per casting (if used properly) is really high. Smite targeting works very well with summoning spells or animated minions. Can be cast in such a way that squares not directly visible (behind a corner for example) are covered. Ignite Poison can greatly increase the damage output (more than double), but it shares no schools with Poisonous cloud and only affects clouds in sight (trees and translucent tiles block it).
As a side note, clouds block other clouds from spawning. This can be useful when trying to get past one of those randomly generated areas with roaming fire and freezing clouds, or miasma, or whatever it is. Poisonous clouds are much easier to deal with, and don't burn your scrolls or freeze your potions.
Superb in Elf and Spider, nice in Lair. Can be extremely useful even in Zot; with some decent levels in the appropriate schools, long corridors, and a little kiting, kills most draconians (green excluded) and even Orb Guardians with minimal risk and low mana cost. As long as you have poison resistance, there's almost no downside to carpet poisoning.
Try to lure enemies into long corridors (especially in Elven halls, sometimes useful in Lair too), then cast Poisonous Cloud into the last spot you see. Consider this illustration:
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Casting this spell on the elves expands the cloud almost to you. As you run away, the elves will have to pass through many clouds of poison, killing them fast.
In a pinch, works great against an Unseen horror. Even if you can't see it, as long as you fill the room with poison it'll die quickly.
All worshippers of Qazlal are immune to their own clouds, and thus do not need rPois to cast this on their location.
Sea of Flame tactic (aka the poor adventurer's Fire Storm)
For poison-resistant enemies, or just for more damage, you can use Ignite Poison to make a sea of flame. Since other types of clouds will block the "spread" of Poisonous Cloud, a simple Conjure Flame in a corridor will keep the poison off of you, and then Ignite Poison will make your enemies stand in fire. Once there, even intelligent enemies will stay in the fire and take massive damage. Also good to deal with enemy summoners and mindless monsters -- it's basically a 9-tile Conjure Flame that takes two turns to cast, and can be cast on tiles with enemies. Add one turn for every 9 extra clouds of fire (just delay ignition until you're done spreading the poison). Jellies and the like run straight through the fire, get wounded, and flee straight back through the fire. Summoners can summon, but they're standing in fire, and so are their summons. Yaktaurs? Standing in fire. Anything that swarms you? Make them walk through fire first. This and Iskenderun's Mystic Blast can get you through the whole dungeon and almost into the Orb chamber, plus any branch that isn't full of fire-resistant enemies.
Poisonous Cloud was removed in 0.20.
Originally a L6 spell, around the time of 0.7 this spell was lowered a level. However, it was subsequently returned to L6 when it was realized that it was overpowered at L5.