For a fantasy writer, 'why?' is the most important and helpful question because it helps him/her avoid writing silly things like:
So a race whose most distinctive feature is "hunger for joy" (hipsters?). That alone sounds retarded but it gets worse. The filthy hipsters don't have jobs but somehow they party all the time in taverns because they have that hunger for joy thing and I guess money grows on tree in Yerengal. However, the society accepted them (how tolerant of them) because the hipsters had a special skill (all of them!). They knew how to make iphones, er, I mean, dur-dur potion which could cure deadly diseases and male baldness.
- hunger for joy is an awful fucking description
- NONE of them - not a single fucking winged hipster - had a job (is there anyone on this team of pros and experts who's older than 12? A wise 14 year old perhaps who has the maturity level of a 16 year old, perhaps?
- even though the fucking hipsters didn't have jobs, they partied all the time, snorting powder.
- but they had a special skill!
So not one of the winged imbeciles wanted to quit doing drugs all the fucking time and maybe open up an alchemy store, selling the dur-dur potions to people?
Here is my advice: when writing fantasies find some historical parallels. For example, the gipsies - they are travelers resisting attempts to settle down, kinda existing outside of the 'civilized' society. They're good at tinkering (a special skill!) and can find odd jobs here and there. Since they survive on their own, they have a wide set of living off the land skills (including taking advantage of the gullible). They aren't all alike (no race is) and many settled down, yet the majority is sticking with their tradition (probably due to their upbringing).
So back to your rillows, while you don't have to explain everything, you have to paint a proper picture. Reading that paragraph creates a lot of questions (and not of the "I want to play the game to find out!" variety).
First and foremost, are they the nation of drug addicts? Do they just brew these potions, sell a batch, then hit the taverns? Is the potion's recipe a secret carefully guarded by the fun-loving drug addicts and not a single addict ever sold it for a few fantasy bucks? Or only rillows can make it, stirring it with their dicks? Not a single fucking rillow has a job? Why? Allergic to non-fun?
It just doesn't sound even remotely plausible overall.
They need to sit down and read some history books just to get the general feel of describing groups of people.
Another thing I find helps with fleshing out races is to start simple, usually their collective desire and even one word to sum them up, and there expand upon it taking it to logical conclusions and then have those lead to other logical conclusions when interacting with other races.
This race described are utter hedonists. They live off of society and only engage in it one way in order to allow themselves to pursue their hedonist life style. Seems they're not very altruistic in the least, they're then very amoral, at least when it comes to others not of their race.
Now instead of having them make potions that cure disease to avoid having jobs, you know, a profession and industry, a job to avoid having a job, have it maybe be a substance they produce from their body, but it's a secret they've kept hidden behind a guise of this potion crap but people spot the discrepancy and it causes speculation (WTF has no one seen a female dwarf), but things are kept quiet because these amoral beings react mercilessly towards anyone who gets in the way of their hedonism.
This last part if you do it not only means you have to change nothing and add a little bit, you can also have an injoke and make fine of the original, stupid, premise that led to this idea in the first place.
That describes them, their outlook and everything, now history: Have them not just being wastrels faffing about, have them be wanderers that came upon a kingdom or continent in the grips of a plague that was literally going to kill everyone off unless they found a treatment for it and these winged things substance was just that thing - being amoral, lazy hedonists, they make a demand to create their society - they have a cure, but they want to rule over everyone for it, everyone has to bow to them or they all die.
So they do and they're the ruling class. In order to keep their order permanent they create a network of servants to enforce their order doling out favour in a business-like manner so those they rule keep themselves in their own place being greedy and narrow minded while they busy themselves being hedonistic. Take a look at the Beglian Congo under Leopold for a historical example.
So you set that premise up, you then already know how it's going to go for later instalments of your game: Their order falls in revolt, and maybe the secret of their "potion" is discovered, it may even be the cause of the revolt. Suddenly they're not simply overthrown, but their empire, full of enraged people, enslave them and turn them into livestock to milk their substance and do it in the most unpleasant fashion possible out of a sense of vengeance.
From this you can now make speculative fiction framed in your world and its history: Third instalment of the series takes place 200 years after the seconds revolution and people don't even know much less care about how these winged beings once ruled over them, they're just winged cows that cure their cancer and they treat them like shit and are utterly cruel because that's the way they've always been treated.
Question: Is this treatment right?
Was it ever right?
These beings are selfish, loathsome, psychopaths, but look at what the other races have done in return. They may just be acting out their nature, but are the other races? They're better than this, but they still keep doing it for reason they don't even know anymore.
Was the enslavement justifiable? The revolution was, but this slavery is no better than what they, and yet at the same time how else could you deal with such a self-centred race, especially one with such a gift as a universal panacea like they have? Even if they can't co-exist with society in a decent way and absolutely refuse to compromise and be equal because they just want to faf about, what else can you do? Is it right to exterminate all of them? Even not simply for their sake but of all the lives their substance might cure? What if another extinct-level disease arises? Is it even smart of kill them all off? If not, then is their enslavement the best compromise that can be made and people just have to treat them more humanely?
As someone said before, once you start digging fantasy, even write period, starts to write itself, but you need to get off on the right foot and not just make a shallow gloss of lore and shit because it WILL look amateurish.