Mercury Vapours

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Version 0.31: This article is up to date for the latest stable release of Dungeon Crawl Stone Soup.
Poisonous vapours.png Mercury Vapours
Level 2
School1 Air
School2 Alchemy
Source(s) Book of Vapours
Young Poisoner's Handbook
Casting noise 2
Spell noise 0
Power Cap 50
Range 3
Flags Not self, Target
Briefly transmutes the air around a target into gaseous mercury, poisoning those who breathe it in. Further, the target and adjacent creatures (living or otherwise) may be weakened by mere exposure to the alchemical substance.

Mercury Vapours is a level 2 Alchemy/Air spell which causes smite-targeted poison on any non-resistant enemy within line of sight, and attempts to weaken monsters in a 3×3 area.

Alchemists start with this spell in their libraries.

Useful Info

Inflicts poison on a central 'main' target, if they don't have poison resistance. It has an independent chance to inflict weakness on the central target and all creatures adjacent.

The main target gains pow/25 (min. 1) poison "stacks".[1] If the target does not currently have poison, 1 extra poison stack is inflicted. If the player is the central target, this spell inflicts 2d6 poison damage instead of any stacks. Creatures with poison resistance cannot be poisoned at all by this spell.

In addition, each monster in a 3×3 area has a chance to gain the weak status, which reduces their melee damage by ×66%. The main target has a (83% - <12 * HD> + <1.5 * power>) * 1.15 chance to be weakened.[2] Creatures that aren't the main target are ×66% as likely to be weakened.[3] Poison resistance or immunity does not prevent the weakness. Keep in mind that you are not immune to the effect.

Higher poison stacks mean more damage per turn, according to the following table:

Average poison damage per 1.0 turn
# Stacks Damage Description
Normal rPois-
1 0.75 1.75 Poisoned
2 1 3 Very poisoned
3 1.25 4.25 Very poisoned
4 2.5 6.5 Extremely poisoned


Mercury Vapours never misses, ignores AC, and is one of the few ways to inflict the weak status. It can be useful even if you aren't into Alchemy. The downside is that it relies on poison - damage is done over time, and cannot damage foes with poison resistance. These shouldn't be dealbreakers, however. You can kite susceptible enemies: apply a few stacks of poison, then retreat a few tiles, allowing the poison to do its job.

At low skill levels, Sting deals more damage/MP, but this spell is perfectly accurate and inflicts weak. In the early game, you may want to cast Mercury Vapours once per monster (to inflict weak and the extra poison), then use Sting. Once you gain more spell power, though, Mercury Vapours is more efficient, so feel free to forget Sting.

Tips & Tricks

  • Ignite Poison is quite strong with any source of poison, including this spell. It converts poison stacks into instant damage, with a great increase in damage. The Ignite Poison + Mercury Vapours combo offers decent damage, high reliability, and good MP efficiency for its level.
  • In Tiles, you can count poison stacks by the number of green poison drops on the monster. 1 drop is 1 stack (poisoned), 2 drops is 2-3 stacks (very poisoned), and 3 drops is 4 stacks (extremely poisoned).


  • Prior to 0.31, this spell was known as Poisonous Vapours. It was a level 2 Poison Magic/Air spell, had full LOS range, but did not have a chance to weaken foes. Also, it inflicted power/15 stacks of poison, but with no bonus against non-poisoned foes.
  • Prior to 0.27, Poisonous Vapours would create a short lived cloud instead of inflicting poison directly. Thus it wouldn't work if there was a cloud already there (including from the spell itself). Also, Vehumet did not consider this spell "destructive".
  • Poisonous Vapours was added in 0.20.