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On each level of the Dungeons of Doom past 5, there is a 1/8 chance that a Leprechaun Hall will exist on the level. Leprechaun halls don't generate ambient messages, so you have to stumble onto it or use telepathy.

Leprechaun halls are filled with sleeping leprechauns and their gold. If you can kill them all quietly, you'll amass a fortune. If they start waking up, though, you'll probably wind up in the hole by a large amount, as the leprechauns deftly relieve you of your zorkmids.

NetHack sadists are known for seeking out Leprechaun Halls with a fully charged wand of death. Pretty sick, but highly effective. Otherwise, magic missile spells or a scroll of stinking cloud will get the job done.