Lurking Rhythmically: The Reversi Puzzle-Trap for Pathfinder RPG

This encounter is a hybrid of trap and puzzle for GMs to use. It's a trap, because of the very real likelihood of severe damage to the party, but it's also a puzzle because it needs to be solved before the players can proceed to the room beyond. In fact, it's the best of both worlds in that the PCs will willingly step onto it and damage themselves for your amusement.

First, make sure you know the rules for Reversi.

Set Up

The puzzle-trap begins with an ornate set of double doors which contain this inscription upon them:

Patience and Balance

Aggression and Greed

They won't realize it at first, but it's advice on how to solve the puzzle (be patient and seek balance rather than capturing the board aggressively and greedily.)

Upon entering, the PCs see a gridded (the absolute minimum size is 6x6, but you can go as large as you want) floor in a neutral color (I used gray) with four floor-to-ceiling columns of primal energy, two radiant and two necrotic, clustered around the central point of the grid. Running the length of the sides of the grid are roiling clouds of positive and negative energy, illuminated from within by bolts of brilliant white and utter darkness. It looks like a a thunderstorm mixed with a matter-antimatter reaction.

This trap works best if there is a door on the other side of the board, for reasons which will be obvious later. There is also a non-board strip of floor running the width of the rooms. If you wish to play up the danger with some flavor, mention that the room is plated in adamantine (unless your players are the types to try to pry up the plating).

I was running "Firehammer Hold" from "Scourge of the Sword Coast and wanted a better puzzle than the lame one in the module. But I had this nifty grid, and felt that chess puzzles were over-used and went with Reversi.

Sequence of Play
The PCs walk in and see the board. Using Detect Magic indicates that a very powerful spellcaster (read: higher level than the PCs so they can't disenchant it) found a way to capture a fraction of the primal powers of creation and entropy. The white columns radiate Conjuration and Evocation, the black columns Evocation and Necromancy, and the clouds are a jumbled mess of all three.

The PCs can mill around off the board all the like. Standing near the fog cloud gives an interesting mix of sensation, like the thrill of an electric charge combined with a chill of utter cold, but does no damage. If they step into the fog, see Entering the Fog, below.

Nothing happens if the PCs step onto the first row of squares. Depending on where they step, something does happen in the second row.

Essentially, if the PCs move into a square that is a legal placement for a Reversi token (they are not playing a specific color), then that token is placed and the tokens between it and its nearest matching color change. If they move into a square which is not a legal token placement, then nothing happens. After that, the board places a token of the opposite color anywhere it is legal to play -- preferably in a way that inconveniences the PCs.

Example of play:

Taszvya, left, takes two steps forward and stops. Nothing happens because that is not a legal token placement.
Perga, right, also takes two steps forward. This IS a legal token placement and the board reacts.
Since Perga stepped where a white token would be placed, a column of positive energy engulfs his square, and therefore him, from floor to ceiling. This also converts the square of negative energy into positive. See Effects of the Columns, below.
Every time a token is placed, movement halts. The board immediately places its own token before other characters may act.
In this example, the board has placed a column of negative energy between Perga and Taszvya. Taz is now in a perilous situation; taking a step forward or backward will count as placing a light token, as will spending a full turn on her current square. If she wishes to disengage from the board her only option is to move diagonally right and down.
Taszvya has chosen to take a step forward, placing another light token. In addition to the columnar effects, the fog cloud recedes one square (as its main purpose is to funnel people into interacting with the columns instead of going around them).
The game progresses as PCs take steps that place tokens and the board reacts. The fog clouds retract as tokens are placed, eventually retreating to hugging the wall.
The game continues until the PCs either leave the room or cross the board. See Endgame, below.
Note: the doors on the other side are locked with no visible means of unlocking them. They unlock automatically as part of Endgame, below, but may be opened by a suitably heroic (35+) Disable Device roll.

Effects of the Columns

It is recommended that the GM keep a count of how many white and dark columns are placed once the game begins, as the effects increment based upon the number of columns placed. (The initial 4 columns do not count toward this total.)

Each time a PC steps into a column, a column appears in their square, or they spend a turn inside a column, the following effects occur.

If a radiant column:

The PC is healed 1d6 hit points for every positive column which has been placed during course of play. Other ailments such as blindness, deafness, missing body parts, etc may be cured/regrown at the GM's discretion.
If the PC is at full hit points, then they must make a Fortitude save with a DC of (5 x Number of columns placed). For example, in step 6 Perga would have to make a DC 5 Fort save, whereas Taszvya in step 7 would have to make a DC 10 Fort save.

If successful, the PC gains 1d6 temporary hit points for every radiant column placed.
If failed, something spectacular happens as their body cannot adequately contain the energy of raw, chaotic creation. Suggestions include: a fireball centered on their location (they receive no Reflex save for half damage on this, although others do); uncontrolled growth; fusion with another PC if they share the same square; creation of a fully-grown duplicate; or other cruel uses of a random magical effect table.

If a necrotic column:

The PC is damaged for 1d6 hit points per every negative column which has been placed during course of play.

GMs who do not wish to kill PCs outright may instead inflict conditions upon them, such as fatigue, blindness, deafness, exhaustion, energy drain, etc. If this is chosen, then require the PC to make a Fort save at DC (5 x negative columns in play) to avoid death.

Any PC who dies in a column of necrotic energy becomes an intelligent undead, such as a wight or wraith.

Bypassing/Disabling the Trap

Players will likely want to skip this whole mess through use of spells. If so, let them. This isn't designed to be impassible; it's designed to be a resource drain for a dramatic encounter later and a source of amusement for the GM.

A few notes:

The columns are floor-to-ceiling, so flying will place a column just like walking.
Intangible PCs will still trigger a column or suffer a column's effects if their intangible form is vulnerable to magical effects.
Teleport, Dimension Door, and other spells will work to get the PCs on the other side of the board, as will spells which send bypass obstacles via another plane (astral, ethereal, shadow, etc).

This is also a potentially lethal encounter for the entire party, especially if they do not quickly realize the rules of the "game". The trap is designed such that a few people can make it across safely, but the more people who cross -- or the longer they dally on the board -- the worse it is for everyone.

Despite this being a trap, there's really no way for a rogue to disable it. However, an epic roll (such as DC 30+ or a natural 20) will suggest a course of action found in Entering the Fog. Lesser rolls can give the following information:

Explain the rules for placement of columns.
Realization that the more people who cross, the worse it will be for them.
Tactics for getting across include: moving to illegal token placements whenever possible; sharing squares (such as smaller PCs being carried by larger ones); or getting charged up on temporary hit points and walking through necrotic columns.
Realization that stepping in already existing columns do not further the progression of the game.

Entering the Fog

Canny PCs will think that entering the fog is instant death. Encourage this, but don't oversell it, because it's actually a way to bypass the entire trap.

Since the fog is made of equal parts positive and negative energy, the net result is neutral, with radiance healing necrosis and entropy preventing explosion from too many temporary hit points. Therefore, PCs who step into the fog are like birds on a high tension line: safe, so long as they make no contact with anywhere else.

Any PC who enters the fog has the Blind and Deafened condition. Further, they cannot be seen or heard by anyone outside the fog. They do, however, maintain their sense of touch, and by touching the wall they can safely navigate to the other side of the board.

PCs cannot enter the fog if it has receded against the wall. Those characters within the fog when it recedes are considered to have stepped onto the board, placing a column if possible.

Exiting the fog by stepping onto the board triggers an Endgame scenario.

Endgame

Endgame occurs under the following conditions:

All entities within the room have crossed to the other side of the board.
All spaces upon the board have been filled.
Someone within the fog exits onto the board.

In the first two situations, perform the following actions:

If a PC exited the fog onto the board, place a column where they stand (if possible) and suffer the effects of same.
Start removing columns in positive-negative pairs.
If all columns are removed evenly (the game was a draw), then the doors on the far side unlock with a loud click and the game board resets to its initial state.
If there is more of one kind of energy than the other, tally how many columns remain. Describe how the energy surges across the board from the fog on both sides (you may, if you wish, give the party time to open the doors/find a way out by having the board fill in the empty squares at a rate of one every round):

If necrotic, roll a matching number of d6 as negative energy damage and apply it to everyone in the room.
If radiant, require everyone in the room to make a Fortitude save at DC (5 x number of remaining columns) to avoid explosion from too many hit points. If they succeed, give them that many temporary hit points.

The doors then unlock.

If you wish to give everyone a chance, have the doors unlock just as the last square is filled in but before the energy discharge.

More/Less Deadly Variants

More Deadly: Have monsters appear through the door or be created by a positive energy surge. Combat on the board ought to be quite exciting. (Energy columns provide a 50% miss chance due to concealment.)

Less Deadly: Use Go rules instead of Reversi and have columns disappear when surrounded instead of flipping.