Blood and souls for Makhleb!
Henzell says...
  1. Level 2 Poison/Air spell new in 0.20. Creates a single smite-targeted cloud of poison gas. Spellpower affects the duration of the cloud, though it is also subject to randomness and will rarely last more than one turn. Found in the {young poisoner's handbook}. Replaces Poisonous Cloud in 0.20. Itself replaced by {mercury vapors} in 0.31.
  2. Monsters that are in a harmful cloud don't mind walking into additional harmful clouds - so combine with Conjure Flame to encourage that ogre to walk into fire.

Information based on master branch and the great LearnDB.
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