Area 7. Sunken Cave, lair of the lich Asberdies (David Sutherland, AD&D module D1: Descent into the Depths of the Earth, TSR, 1978 by Gary Gygax)
There's something perfectly old-school about this encounter. Hidden behind an illusion is the lair of a lich with a +3 ring of protection and numerous spells up to level 9 to hide himself, stop and disarm an overly curious party, summon monstrous allies, and escape unharmed if needed. And if the party persists in trying to locate the source of the magical effects,
"The lich, Asberdies, has cast 600 magic mouth spells in various portions of his lair -- walls, floor, ceiling, and on stalactites and stalagmites too. Therefore, magic detection will show virtually every place in the cave as radiating magic."
It is frustrating that Gygax did not mention what the mouths are programmed to say. Perhaps he meant them to remain silent. Supermodule GDQ1-7: Queen of the Spiders (1986) describes Asberdies' tactics when defending his lair, beginning with a warning delivered through ventriloquism, which more logically could have been spoken by one or more of the mouths.