Little idea I had at work: for GMs comfortable with improvising:
The demon lord Anarazel delights in greed, bloodlust, and recklessness that adventurers so often fall prey to, and so fashioned these items to give them endless opportunities to practice said vices. They appear as small coins of an unknown black metal, polished to a mirror finish. If a word is spoken to one, and it is buried in the earth, some time later an entrance will emerge to a dungeon themed by the word in question: “fire” will produce a fire dungeon, “spiders” will produce a spider dungeon, etc.
Most coins produce relatively small dungeons: a single level with 5-7 rooms in them. Roughly 1 in 6 produce greater dungeons: 5-7 levels deep, and 1 in 6 of those (or 1 in 36 overall) will produce megadungeons consisting of 5-7 full dungeon “branches”. Small dungeons will take a few minutes to “grow”, greater dungeons will take a few hours, and megadungeons will take a few days. Almost all dungeons will result in thematically appropriate monsters, traps, tricks, and treasure. Usually, any treasure will be commensurate with the risk involved: a “harmless” dungeon might only have a few copper coins scattered through: contrariwise, attempting to cash in with the word “gold” might result in a dungeon filled with lethal golden constructs and deathtraps.