Monster A + Trap B + Spell C= Disaster.

The first step  is asking oneself, “How contrived do I want to make this?”

Allow me to give an example:

GM Jimmy wants to really mess with his fantasy TTRPG players this evening. The monster is cleverly “mimicking” a chest at the end of the long hallway. Thus it appears that loot is readily in sight. The stone floor of the hallway is shiny, but otherwise unremarkable. The walls are flat and smooth. All is normal until an adventurer walks the first 3.5 meters down the hallway. The stone floor is actually on a large pivot and the rest of the party is boxed off by stone. Meanwhile, the first adventurer in the hallway discovers that the floor is magically greased, and he is now sliding toward the open mouth of the gruesome tentacle-teeth monster that was a chest just moments prior.

We have a tentacled maw with the ability to appear as other objects, the pivoting stone floor trap, and a magical grease spell on the floor. Two out of the three might be detectable. The monster maybe not so much until the group gets close enough to see the broken bone, bits of cloth and scattered debris behind the monster.

That’s actually pretty tame compared to some of the ones I’ve come up with or had used on my characters. It creates moments when the group is happy if a room in the dungeon was empty or if all they ran into were a couple of skeletons. Surviving combos can give dungeon savvy groups something to talk about for weeks and months to come.

Personally I’m fond of an Illusion, a trap that probably could have been avoided except the party was preoccupied with the monster. Illusory walls and floors are pretty cool. Also, spell casting monsters make some of these situations especially nasty. If an illusion is used as bait, the player might be a bit salty that she survived the rest of the encounter for it. I try to at least leave somewhat of a reward, even if it’s not the one they thought they were getting.

We should talk at length about illusion magic sometime. There’s also various plant, elemental, and paranormal environmental effects that function like traps. Hallucinogens are fun if we don’t over do it. (In game. IN GAME! Don’t do that stuff in real life. I would not condone that.) Sometimes when the characters are seeing things, they don’t need the rest of the combo to really make a mess of things. (Evil, maniacal LOL!)

Keeping this one kinda short because I have a cold. My plan to cover the Dungeons & Dragons monster met with trademark issues. I mean, I could just rename the monster, but I don’t trust that it won’t boil over into something. I just don’t trust Wizards of the Coast. Hopefully this cold will pass rapidly. Until then I’ll just keep doing the best I can.