Magnets are super fun in both real life and in DnD. For this trap example, we use magnets in a creative way to get your players to look for obvious traps when the signals are present.
The Magnets Trap
This trap starts when the players enter a room with a sloped floor that angles down to a door on the far wall. On the floor, the players can see many mundane weapons laying around. None of the weapons seem to be anything of note, but they are all metallic in nature. When the players enter the room, nothing appears to happen. Unless they inspect the ceiling, nothing will seem amiss until they trigger the trap. Once they try to open the door at the bottom of the ramp, a cylinder descends from the ceiling and magnetizes by any method you wish. This magnetic cylinder picks up weapons as it rolls and quickly becomes a whirling collection of blades and maces!
Perception for this trap should be easy, but the mechanism not obvious. The weapons all over the floor should tip off the players. The adventurers may assume the floor is the trap and be cautious entering the room. You should give them hints based on their perceptions rolls. A high roll would allow them to notice the cylinder in the ceiling, but may not indicate how it works. The handle of the door triggers the trap, so it would not be obvious either. You can reward players that roll particularly well by informing them the door has no hinges, which may indicate to them it is a false door.
Triggering the Magnets Trap
The only trigger for this trap is the door handle. It’s a mechanical touch trap, so magic detection will not help here. The best way to avoid the trap is to be wary of it and notice either the cylinder in the ceiling or that the door has no hinges.
Damage for this trap is bludgeoning or slashing damage and should increase as the cylinder and players get closer to the bottom of the slope; the more weapons the cylinder picks up, the more dangerous it should be. This damage can be scaled however you like, but we always recommend that the damage should never be enough to outright kill someone in your party. For example, you could increase your damage dice by 1d6 every 10 feet the device moves.
Disarming the Trap
The trap can be disarmed mechanically by disassembling the door handle, but it should not come without risk. Players may need to place themselves in harm’s way in order to disarm the trap. The trap may be overcome once triggered by having players work together to stop the cylinder, or by putting up an obstruction in its path. The DC should be determined based on your party’s average strength.
Appropriate Trap Use
This trap is simple and fits in almost any dungeon. Its easy to use nature and lack of reward make it a great trap for low to mid level dungeons. It can be quickly used to show that any area can be trapped or to teach your players not to push their luck in an area. While it can be punishing if used wrong, it’s so simple and cinematic that there is almost no wrong use for it. The more players you catch with it, the easier it is overcome since they can attempt to stop the cylinder as a team. Try it out with your group and see how they react to it. Happy DMing!