Four Against Darkness simulates a four character party exploring a dungeon on a quest to slay one particularly nasty boss monster. On each turn, you randomly generate a dungeon room that gets added to your existing map. We then randomly generate what your characters discover in it, and if it is monsters, you have a chance of negotiating (and losing the initiative if the monster is hostile,) or taking the first strike. A system of discovered clues and friendly NPCs allows additional goals to emerge during play, and possibly add a campaign element to the way the game is played.
At the end of the day, it's not a game with a great deal of depth: You explore the dungeon, you slay the monsters, you loot the room. Sometimes the monsters ask you to kill a different monster. Sometimes you find a weird puzzle, or a clue that will eventually allow your characters to score an artifact. Its a simulation of 5th edition beer and pretzels game. And sometimes, that's all you want.
Not to mention the fact that it is awfully handy for testing the mechanics of other games.
Shortly after I received it as a gift I started playing it on my laptop using Keir's Dungeon Scrawl. A favorite program of mine that I've used for dozens of projects, personal and professional.
At one point, my son climbed up onto the couch next to me while I was playing a round, and instantly figured out that this was some kind of role playing adventure game - he knows a dungeon map when he sees one - and asked if he could play, too.
This strongly resembled a heartwarming Twitter post I had seen a few days earlier about a dad who had turned Four Against Darkness into a game he played with his son. And it doesn't hurt that for against darkness has suggestions for how to convert it into a rules-light light RPG, either with or without a GM.
So, we generated some new characters, and began exploring dungeons together.
The engine for Four Against Darkness is even simpler than the engine in Tiny Dungeon. I would say the two are neck-and-neck, along with Minimo for the distinction of being the lightest role-playing games I have ever seen. Four Against Darkness is slightly crunchier, in terms of math, and a little more Lo-Fi at the same time. I would say my play preference lies more with Tiny Dungeon 2e on that front. Which is fair: it is designed to be played in a group. 4aD isn't.
However, something about that particular engine really locked in for my son. And, when he wants to run a game of his own, he often uses it as the core engine for his improvised TTRPG, minus the dungeon mapping.
It has also been a great tool for teaching him Math at home, as it involves a little addition and subtraction of numbers under 20. And taught him how to read maps.
If you are looking for a GMless game that requires some technical thought, you can't go too wrong with it.