First off, I'd like to give credit where it is due: I was watching a walk through of Jak and Daxter 3, when this occurred to me. Jak gets many special ability that lets him access new areas. The one that triggered this particular idea is where he turned into Dark Jak and was able to blow away an alcove to reveal a hidden area. If there weren't video game mechanics involved, such a thing would "rehide" itself after his passing.
I was thinking about having a cult in my game, the populace controlled or manipulated somehow, and on command they go to a secret area or specially designed demiplane. The only way to get into this area is to have some sort of mark, or special ability that lets them pass through (though touching someone with said mark would allow others to get through, but no one with it would willingly allow this because it would expose the secret). I was thinking of 6 criteria for this:
No Level: Whatever's controlling them can't have a level requirement
No Theft: It can't be stolen (whatever it is is innate in the user or is passive in nature to those so controlled)
Nothing Obvious: It can't show up as magic or technology with casual investigation (it might be magic, but would need something a bit more specialized than "detect magic" / "detect evil" or if it is technology, it can't be visible).
No Direct Control: It can't show the method of control directly. (more investigation by the PCs will be needed, but with a bit of luck and skill, some clues laying around will allow them to solve the case)
Willingness: It needs to be done willingly - though coercion/social pressure/trickery/physical sickness, exhaustion, etc may be involved to get them to agree to it (therefore the "victims" of said control / manipulation can't said to be forced into taking it (at least not directly)
Another Benefits: Someone else (the BBEG or a mini boss) is able to benefit from the control either directly or indirectly
Long Term Effects: (Bonus, but not required) if one or more of the PCs willingly accept the mark to get the others in, it'll have long-term consequences in the game beyond this story arc.
I am looking for personal experience achieving this kind of effect: how did you define the effect (monster effect? spell? magic item?), what rules did you use, how did it work out?