I once had a campaign world set in about the 16th Century in which Fey played a big part.
The Fey World (Arkady) was a world built of 'Phantasm' an actual substance generated from the dreams of mortals. In fact Arkady itself occupied the realm of the collective dreams of mortals. There was thus two ways that the Fey could come into the Mundane world
1. by accessing mortal dreams - which required a lot of power to do so and which was greatly discouraged; and
2. via Trods - places of magic where the mortal and fey worlds intersected (the usual method of crossing over)
The Fey world being generated by mortal dreams has many similarities to the real world in terms of landscapes etc, life for the fey continues much as it does in the mortal world as individuals go about their lives and the Lords and Ladies plot and scheme.
However the landscape though similar to the mortal world is not at all stable - they are created from Dreams and Dreams can change them. In addition the Lords and Ladies themselves have the power to shape the Phantasm and so can change the environment to suit themselves.
The Seelie and Unseelie courts are not fixed entities either but rather refer to traditions which might be followed according to preferences and peculiar likes of the ruling Lord/Lady. Remeber though that the Fey are fickle and a Lord/Lady could go from being Seelie one day to being Unseelie the next - the Fey are dangerous and not to be trusted.
Anyway an Unseelie Court is one inspired by and built from nightmares- the Lords and Ladies of such a court like to inspire Fear and extend Insanity. Decay and Disease are to be celebrated and terror to be enjoyed.
Seelie courts on the other hand are inspired by more pleasant dreams and so take on the wonderous aspects often viewed in mortal art and childrens dreams. A place of sunny days and golden feilds where the inhabitants laugh and play. Wonder is to be enjoyed and joy celebrated.
Despite the nature of the Courts all Fey are mischivious, hedonistic and selfindulgent. Because their world is so entirely mutable so are they, they are fickle and changeable and a Fey who is often friendly and polite can quickly become hostile and devious. - Again never Trust the Fey!
Elves
Elves are 'Sidhe' in ALL my games. I take the base elf make them Fey, make them Phantasms and able to use Obscuring Mist at will and give them a mysterious alien psychology. They are NOT a PC race and usually avoid mortals (unless they are goign to kill them)
Mortals in Arkady - Jacks, Clods, Wits and Changelings
Jacks - Jacks are mortal champions sometimes called in to assist the Fey in times of need. (Most mortal PCs working in Arkady will be Jacks)
Clods - a Clod is an unfortunate Mortal whose mind has been bedazzled by the Phantasm. They can see into the Fey world but are usually also quite insane.
Wits - a Mortal who has been given Fey knowledge or a Fey-gift (like Fey music)
Half-elfs - IMC they are Half-Fey born from the union of Fey and Mortals
Changelings - A mortal baby taken and raised by the Fey of Arkady. Changelings being mortals have the ability to dream and this becomes a huge power source for the Fey. Adult changelings who are not returned to the Mortal realm become Erl-Kings powerful beings able to create and shape Phantasm (and the world around them) at will
* In my campaign the villian was an Unseelie Erl-King who had found away to control the minds of mortals and threatened to conquer both Arkady and the Mortal world...
Other Figures
The Mother (called variously Mother Goose and grandmother) is a powerful primordial being said by some to be Mother Nature herself. She is a protector of children, the first hag and even though she seems matronly and kind all Fey fear her wrath.
She is attended by 7 Sisters, all powerful Sidhe who are the 'Fairy godmothers' of folktales. They watch mortal life and report to the Mother of all that happens.
The Mother also has a mortal priestesshood whose role was once that os Shaman-healer. As times progress they have lost status and have been degenerated to 'Tooth Fairys' because of their role in collecting the teeth of children which they deliver them to the 'Mother' (because a Fey could use a mortals teeth to control them and thus their mind and dreams)
