Demon cartographers (not specified if this would be a new monster species or just the occasional random peaceful monster of an existing species) who offer to sell you scrolls of magic mapping for exorbitant prices.

“Partial” magic mapping from a cursed scroll maps a contiguous chunk of about 1/7 of the map, rather than random tiles.

If blessed, the scroll of magic mapping reveals the vibrating square.

The confused scroll of magic mapping tells you where some special level or branch entrance exists on the main branch of the dungeon which you have not yet visited. If you have visited every special level including the Sanctum, it gives you a message about the DL 1 upstairs.