A mechanic for fantasy campaigns in which mortals venture into faerie-land to do their adventuring.
Attunement starts at 1 for humans and orcs, 2 for other races and wizards, 3 for Elves, and 4 for actual faeries. (The GM could even have players roll 1d4 and select a race/class based on their result.)
With each full day spent in the fey realm, all player characters gain 1 Attunement. With each month spent outside it, they lose 1. Some faerie magics such as resurrections or "blessings" from powerful sources can also increase Attunement by 1.
In place of Reaction rolls in faerie-land, players must roll 1d6 under their Attunement. The fae courts value etiquette in their own alien manner, and a good first impression can be vital.
If Attunement ever reaches 7, the character can never leave the fey realm.
If Attunement ever reaches 0, the character forgets faeries ever existed, and can never find their way back to faerie-land again.
Optional Rule: Roll-over Attunement in place of reaction rolls when in the mundane world. On a failure, the character is thought to be drunk, ensorcelled, or mad.