You’re GMing an RPG game tonight, and you haven’t done any prep. Normally I use the Lazy DM checklist or the 7-3-1 technique or hexcrawl prep, but when I’m crushed for time I use what I call Monty Hall Prep: I read up on 3 different monsters, either from the Monster Manual, the bestiary of the system I’m playing, or these days from Keith Ammann’s book or blog. I write down 3 “doors” (locations) to choose from, foreshadowing but typically not naming which monster is “behind” each door. In play, I’ll give some details immediately, while others will require further investigation.


  • Recently the PCs were traveling out of a swamp back to the capital. I decided an unnatural fog would fall when they passed a graveyard on the edge of the swamp, obscuring their vision, and presenting the three “doors”. “You see a lantern off to the right [will-o’-the-wisp]. From ahead you hear the clops of a horse’s hooves [Black Rider]. Now you hear a woman sobbing in the graveyard [banshee] and speaking, in words almost recognizable.”
  • In another session, I doodled caverns exiting the chamber they ended the last session in. One is natural limestone, carved by an ancient river, now full of stalactites and stalagmites [one of which is a roper]. Another is an old mineshaft, little used, and cobwebs cover rusted equipment [giant spiders]. The third is an ancient tunnel heading down, square and true representing the finest of ancient dwarven craftwork; PCs can hear murmuring and shuffling echo up from the depths [might find bits of decayed flesh from the zombies].
  • In one of my campaigns, the druid was always shapechanging into a bird and scouting out everything. So I used less foreshadowing and more mysteries or hidden enemies. Example— To the northwest, the end of the mountains, the most direct route to the capital, is a hydra and elsewhere bands of kobolds on the move [unseen: winged kobolds]. In the pass PCs have taken before through the hills, feasting on a sandworm is a band of ogres and another creature [a humanoid with 4 trunks, trunks like a wooly mammoth but hairless: a mindflayer]. To the east is a pass between the hills and mountains that leads directly to the swamps southwest of the capital: two hill giants, each with a wolf, on the prowl.
  • The imprints of the feet of a giant [stone giant], heading upland into rocky ramparts lined with caves and cavities towards a carved entrance among the openings, like a mine entrance. A winding path down out of the mountains, with a nice view of the lake, exposed to the sky [risk of the wyvern they had seen last session]. An ancient winding stairway chiseled into the side of the mountain, curving up out of sight (elvish work; ends abruptly in rockfall); the baying of goats from up the stairs [satyr].

The lower the prep, the greater the need for improvisation. For the fog scenario, the PCs ended up investigating the graveyard, and I improvised the banshee as an elvish maiden, part of a love triangle, and how the PCs could put her soul to rest. For the banshee’s backstory, I kept open the Wikipedia article and the Monster Manual entry on banshees in D&D Beyond. I ended up leaning on my past hexcrawl prep, which had some details about the village, and relying on my pre-generated list of common NPC names. When stumped, I asked the PCs questions and integrated their answers into the story. Their tactics also provided clues on what they thought had happened.

As GM, you can also leverage the unopened “doors” as appropriate, especially if using a PbtA system where failure leads to hard moves against the player. The PCs in the fog ended up leaving the banshee to go search the village for clues, and I could have had the Black Rider approach and act as another threat.

It’s always humorous when the players pass on any of the “doors”. Initially, the players decided to make camp in the fog and wait for it to pass! I definitely should have had the Black Rider approach and force the choice. In the choice of caverns, one of the PCs investigated a deep pit in the room, which had been described the prior session; this was a 5e campaign so on failed investigation checks nothing happened; had this been Dungeon World, the zombies or spiders might have attacked.

Monty Hall Prep has become a valuable tool in my toolbox as GM, and I hope you’ll find it helpful. Let your players find out what’s behind door #3!

Image placed in the public domain by Cepheus.