These rules are for use with the Underneath system, which makes use of a sanity mechanic to potentially have different player-characters seeing the same thing differently, simultaneously.
It's assumed that we're working within the horror genre here, although I suppose the same arbitration/narrative issue will crop up when dealing with any sort of diverged realities- be it hallucinogenic drugs, magical illusions, or whatnot.
When you're at the table, one of the unspoken assumptions is that all of the player-characters are essentially experiencing the same reality. This is derived as a matter of convenience - one Game Master, one reality. Having multiple realities simultaneously present necessitates a kind of narrative juggling which can be difficult to manage.
And in my experience if two realities are being experienced, the players will poke at it until it stops or they give up and leave. This can be difficult to narrate and reconcile. So here's a procedure for Underneath to help you as a GM.
Group A experiences "Reality A", Group B experiences "Reality B". How do you resolve this difference? Do you resolve it?
There are four possibilities for how it can resolve:
- Reality snaps to A for everyone.
- Reality snaps to B for everyone.
- Reality remains differentiated.
- A Synergistic Reality - A-B - comes into being.