A bare or sparsely furnished room with two distinctive features: The first feature is a solid iron door with a large recessed gear located in the middle. There are no handles or keyholes visible and the hinges are set on the opposite side of the door, indicating that it opens inwards. The door cannot be broken down and is constructed of an alloy that will not yield to magical manipulation. Also, due to the lack of any conventional locking mechanism, it cannot be picked. The door is framed in stone which has been decorated with plates that have been hammered directly into the stone to form a simple geometric pattern (see below), part of which is comprised of a bone-like shape.
The second feature is the floor which is covered in metal tiles, each in the shaped of a stylized, spiked skull that forms a regular tessellation. The eyes and nose are filled with small, dark triangular pieces of metal that are hinged to the tile along one edge - a detail that is not observed under normal circumstances.
Unlocking the Door
The door can only be unlocked by turning the gear. To accomplish this, there are two tiles that differ from the others which, once located, can be picked up from the floor and placed - end to end - in the gear. These key-tiles are missing the triangular piece of metal in the nose, and are cut off shorter than the other tiles. To hide this fact, the tiles are placed against a wall or other flat, perpendicular surface. Exactly where in the room they are located doesn't matter, but practicality suggests they would be close to the door.
In addition to the two tiles, one of the metal shapes in the door frame is actually a handle that can be pulled loose - a fact that can be discovered upon close inspection. The handle is then fit into the two nose openings of the key-tiles, allowing them to be turned, unlocking the door.
|Above: Layout of the doorframe pattern and floor tiles. Suggested locations for key elements are shown in red.|
Any melee or lightning based attacks that strike the floor not only cause lighting damage to any creature standing on the metal tiles equal to half the value of the original attack (Ex: A Wizard casts Shock Sphere on a section of the tiles and rolls 10 damage for the attack, any creatures/individuals standing on the tiles will take 5 lightning damage).
Such an attack will also cause the hinged metal triangles to flip open, point up, for the remainder of the encounter. While open, the metal spikes are considered difficult terrain. Any movement across the floor will result in an level-appropriate Acrobatics check. If failed, the individual takes 1d4 damage and is slowed (save ends).
DM's will probably get the most mileage out of flying/hover creatures as well as any that can use a lightning-based attack from a position of relative safety. Grell Philosophers (MM pg. 144) hover and can cast lightning, making them excellent choices.
Another approach would be to use a trap that does lightning damage - such as a crystal mounted on a podium in the center of the room - to activate the floor spikes. The podium could be raised but hard to stand on, requiring an Acrobatics check while attempting to disable or break the trap.
The style of construction is grim, but displays the kind of detail and efficiency found in Dwarven stone- and metalwork. However, enemy magic-users with even a moderate level of intelligence could be expected to figure out how lightning attacks activate the floor trap.