Kill them all!
Henzell says...
  1. A cloud produced by {bat swarm form} and {vampire bloodprinces}. Living entities (players and monsters alike) staying in them take minor irresistable damage from the bat bites and become Drowsy; after staying in them for for multiple turns, they fall asleep.
  2. <regret-index> Vampires specifically attacking people in their sleep is quite common in fiction, but Crawl unfortunately is not a context where the player can invite each monster to stay over in their mansion for the night, and requires more proactive effects. <DracoOmega> Assume the bats' saliva is a sedative or something.

Information based on master branch and the great LearnDB.
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