Recently, I started prep work on a West Marches style D&D campaign in a custom world, and that’s had me facing all kinds of new challenges. I’ve been messing around with hex maps in Worldographer and hand-drawn maps for players, I’ve been raiding every D&D 5e supplement I have for interesting monsters, and I’ve been trying to piece together cool locations for players to visit — especially dungeons.

While I was reading a Reddit post on unique encounter effects, I remembered some of my older attempts to bring mirrors into combat. I’ve always enjoyed the magic of reflections — duplicitous creatures project images in mirrors, wizards and demons capture creatures in them, and of course Vampires don’t have reflections. In myth and fantasy, mirrors are made of truth and lies. So here’s my take on an encounter built around one.

The Monster in the Mirror

The players enter a large, rectangular room with doors on opposite ends. It’s barren, save for a pillar in each corner and two stone pedestals, each set 10 feet in front one of the entrances.

If any player tries to cross the middle of the room, they encounter an invisible, solid barrier that extends across the room’s width, preventing passage.

The pedestal on the near side of the room bears a crude carving of a humanoid silhouette standing before an empty doorway. There’s an inscription beneath the image:

From one world to an other,

Transients pass and eyes deceive;

The thing beyond the veil

Is truer than it seems.

When this text is read aloud, the party may begin to feel as though they are being watched. The players should be given a brief moment to speculate, but before they act, the door behind them seals itself shut as a creature enters on the opposite end of the room.

You can describe this monster however you would like. I prefer the idea of using some kind of spectral human or a writhing mass of shadows, but you might choose to describe something corporeal. Roll for initiative.

The monster that the players see is actually a reflection of the true monster; the invisible barrier in the middle of the room is a mirror to the Ethereal Plane, and the real monster is on the same side of the room as the party. Since the player characters exist on the Material Plane, they don’t cast a reflection.

Initially, if players attempt to attack the monster across the room, they will find it to be impossible. Players can’t cross the room, and projectiles and spells harmlessly bounce off of the invisible barrier.

When the invisible monster moves to attack a player, its reflection will move to the opposite location in the room and it will appear to swing at nothing.

Try to give your players context clues where possible. Let them bump into the invisible creature if they pass into its space. Let them make perception checks to recognize that the far end of the room seems impeccably identical to the side they are standing on. And most importantly: once the players recognize that the monster they see is a reflection, let them use it to figure out the real creature’s location so they can attack it. Their attack rolls may still be made with disadvantage since they have to look at the mirror to know the monster’s location, but give them freedom in how they fight back. Reward creativity.

You can use any level appropriate invisible or ethereal monster for this combat, like a Poltergeist (Monster Manual 279), Wraith (MM 302), or an Invisible Stalker (MM 192). Just be aware that it might take your party a round or two to figure things out, so this could prove to be a difficult fight.

If you want to make things even harder for your players, allow the reflection to switch places with the real creature by walking through the mirror so that it can tag-team against the party. Just have both creatures share an initiative, and ensure that only one monster is ever on the side of the players.

Once the party is done kicking ass (or clawing their way through a hellish encounter) the mirror disintegrates, revealing a hidden passage or a treasure chest. If your party wishes, they can collect some of the dust from the disintegrated mirror. You can use this as a crafting reagent or a magical item of your choosing, like Dust of Disappearance or Dust of Sneezing and Choking.

Feel free to let me know if you have any suggestions for this encounter! Remember that D&D is all about having fun and telling your own (collective) story. Take whatever you love and build it into your games. If you like this encounter, use it. Change it however you’d like. I hope it serves you well.