I am very fond of wandering monsters. This is partly due to me still running large dungeon complexes, but I also use them in wilderness adventures to break up the play and provide moments of unscripted action.

The original version of wandering monsters in the dungeon was a check every 10 minutes, with a 1 on a 1d6 roll indicating monsters encountered by the party. It’s worth noting that this ten-minute period (known as a “turn”), was the period used to break up exploration. In a turn, you could have one combat, explore a certain section of the dungeon, search one room, and so forth.

It’s a very useful period of time. When searching for secret doors and treasure caches takes a turn, and you also make a wandering monster check, there exists a tension between how diligent you are and the risk you take.

In 5E, that 10-minute turn also becomes the length of time to cast a ritual spell.

Of course, just because the party meet wandering monsters, that doesn’t mean the encounter has to be hostile. Original Dungeons & Dragons has a reaction roll to see how the monsters react to the party. The version I like the most is an easy 2d6 roll, with a 2 meaning hostile and immediate attack, with 12 meaning friendly and helpful, and values in between ranging the spectrum.

I’m terrible at making friendly encounters though. I want a combat!

I like it better when I can give more context to the encounter than just “the monsters are wandering around the dungeon”. This typically requires you to know where they live in the dungeon normally and what their lives are like. If you encounter goblins in the corridors, why are they there? Are they looking for food, visiting friends, on patrol for intruders, or something else?

A lot of wandering monster tables are just something like “on a roll of 1, the party meet 1d6 goblins”, which is totally fair when you’re in a single-monster complex. The more detailed tables – with perhaps 20 different monster types – presume a larger dungeon, perhaps with links to the Underdark. Thus, not only are monsters appearing that live in the dungeon somewhere, but also have entered from the far greater unseen world beneath the characters’ feet.

The challenge remains in giving them a reason to be present. Getting past the new DM tactic of “they’re just there to fight” to providing more context to the encounters isn’t something that comes naturally to everyone. Honestly, you could also say that for just regular encounters in the dungeon environment!

How tough should a random encounter be? Now, that’s an interesting question. In some dungeons, these random encounters are primarily there to give a sense of movement and life. So, easy encounters that don’t disrupt the regular play. If there’s combat, it’s over quickly.

In a dungeon where the keyed encounters are very interesting, that’s sort of what I’d move towards.

Meanwhile, in other dungeons, I wouldn’t object to having potentially deadly random encounters. Perhaps you think the players should keep on their toes, or you want them to try negotiating from time to time, or that it just gives more of a feeling of a dangerous world.