I don’t consider myself an expert on Unseelie, but here we go. 🙂 I think to start though we need some proper context.
In the beginning there was just the fae courts. They were unified and while there was sometimes conflict there was peace. Then stuff happened, alot of stuff that is tl;dr for this post and I don’t even know all of what happened, but, tl;dr, there was a difference of opinion on what constituted the right to rule.
Generally who we call the Seelie today (note, these are mortal words anyway) believed that the right to rule came from birthright. Passed down from those who already rule. Those generally considered Unseelie believed it came from strength, both from strength to assume leadership, but also strength to defend it. Still other courts believed the right to rule came from the those who would be goverened, who had the most shinies, etc, but those principles formed courts that would not see so much attention. I’ll save those for a later time.
Anyways, the terms Seelie and Unseelie come from Gaelic, and are not necessarily of fae origin (even though we use them). Seelie, meaning happy, holy, pretty, shiny; and Unseelie which means sad, unholy, ugly, and dim. It is a distinction that mortals made that we fae did not (until later when mortals started referring to the courts this way).
With that context, I think I can talk about Unseelie.
As noted they are the the type of fae that believe the strongest one wins. Most of the types of fae that are Unseelie are ones who are known for killing, whether its mortals or other fae, that does not really matter that much to them. They are also likely to employ fear to get what they want. But they also have a kind of honor around their bloodlust. An Unseelie is not the type to pussyfoot around with poisons or backstabbing or other methods of subterfuge when they decide to kill.
Their court operates much like this. Subduing your superiors is the way to get a promotion, their kings and queens stand upon the mountains of those they had to conquer and eliminate to get there. Sometimes this involves murder, but the Courts are usually more .. well .. courtly than that, and simply beating them to a bloody pulp is enough to prove your point.
You might think that this would create an environment of blood and ugliness, but in their own way, Unseelie castles and kingdoms have their own dark beauty. Things are kept clean, castles are built, etc. But the Unseelie Court, when it is run, really just gets down to business when its halls open, without all the pomp and etiquette (though for sure there is still, etiquette) of the Seelie court (which is almost etiquette for its own sake).
Being part of the Unseelie court has more to do with what you believe you are than things like birth, race, and personal edginess (heh). In most cases if you want to be Unseelie you just got to kill another unseelie (or beat them thoroughly) or eat them or something (assuming you’re fae yourself) I don’t think any of them would argue with that.
Because of this (also much like the seelie) there is no specific “racial associations” with being Unseelie. There are unseelie pixies, and sidhe, and even unicorns.
I think the last immediate thing I can think of, is that if you want to meet unseelie fae out there in nature, you can call to them and/or look out for them, but if you meet any, respect is helpful but not mandatory, but do not do anything that might be construed as a challenge, because if you do, they will accept and hold you to it. If they challenge you in some way, you may respectfully decline, but expect them to deal with you no longer (at least for a while, and they aren’t a collective, so there are still others to talk to). If you accept it, you might be harmed, killed or eaten, but if you win, its worth *alot* of credibility and respect with the Unseelie. But I should reiterate, this can be quite dangerous.
EDIT!: @unseelie-sorceress is an Unseelie and more informed regarding them than I. You should also ask her.