For those that value my thoughts here they are.
The dark kingdom was once called Lunaria. It snows most of the year, with only Summer letting the ice melt. Autumn is when it starts to snow again. The melt only happens down south. It's legit the coldest kingdom.
Lunaria was the sister kingdom to Saporia, and back in the old days they had a very close relationship.
Lunaria's capital was actually further north. The current castle used to be the temple of the moonstone.
Saporia protected the sundrop the same way Lunaria protects the moonstone. They had a temple and a seperate capital.
Opposite to Lunaria, Saporia had much warmer temperatures. In some areas, being without shade can result in burns within a minute of exposure. They are across the sea from Corona and basically live in a desert. Kind of like Egyptians. Around water (which there was plenty of in certain places) their was a huge abundance of life.
Lunaria and Saporia, due to their close bond and protectiveness over their artifacts, refused contact and treaties with the other kingdoms. Their trade routes with each other were highly fortified and guarded.
The seven kingdoms got angry with this, and decided to wage war against them. The kingdoms also wanted the power of the Moonstone and Sundrop. Saporia, despite Lunaria's aide, eventually fell to Corona and the love between Herz de Sonne and his wife. Then the kingdoms focused on Lunaria.
The Lunarian King basically ordered the prince, his wife, and the civilians to go to the temple of the moonstone to fortify it. They would make their stand in the capital.
The old capital fell, but the Lunarian King used magic bestowed by the moon to curse the land. Only Lunarian's can survive in it.
The curse basically caused most areas of Lunaria to have a climate event called "Flash Freeze". Basically, periodically, the air suddenly becomes so cold that people without Lunarian blood end up becoming Flash Frozen. The old Capital is in one of those zones.
The last of the true Saporians protected the Sundrop with their lives, leading to them relocating the flower out of the ruins. Now the old Saporian Capital and the old Sundrop Temple remains abandoned.
The Moonstone Temple eventually became the capital and castle. The Lunarians made new villages outside of the decay so farming was a thing.
Yes the whole Kingdom is not decayed. Outside of the radius of decay, the land is snowy and mountainous.
The Lunarians are the people of the Moon and thus, praise the moon and worship her. They are nocturnal, with natural night vision. The moon, who is sentient and operates through the moonstone, deeply cares for them.
This is the same for the Saporians. They worship the sun and are legitimately impossible to blind. The sun, through the sundrop, loves her people. (This did not translate through to Rapunzel).
The day time is always cloudy in Lunaria. However, as soon as the sun sets the cloud dissapate and the night sky is absolutely beautiful. You can see every star and a bunch of galaxies. There is always a blue/green aurora borealis in the air.
It is considered super offensive and rude to approach Lunaria during the daytime and try to interact then. Only go during the night.
Lunarian and Saporians both have a native language that is different to the common language of the Seven Kingdoms. Their languages are similar to each other so they can easily converse, however it is very different to the common language.
Lunarians will, at least once in their lives, go "Moon Mad". During this the moon whispers to them to try and calm them down. This happened to Edmund and is why he tried to destroy the Moonstone. The Moonstone reacted to him as if he were someone trying to steal her away and blasted him back.
Varian's villain arc is the result of Moon Madness, explaining why he bounced back so quickly.
Saporians have their own version, called Sun Madness. Same deal.
Andrew experiences this.
Andrew: Go big or go home. || Sun: I'm begging you, go home. For once in your life please, just go home. || Andrew: I'm going big! ||
Creatures in Lunaria are basically bigger and deadlier. An example is Ruddiger. He is not a Coronan Raccoon but actually a Lunarian Coon. The potion that "Mutated" Ruddiger actually just grew him up a bunch. He is a baby rn but will eventually grow into that giant and dangerous beast. (He'll still be a cinamon roll tho].
Lunarians and Saporians can sense if someone is Saporian/Lunarian by looking at them. This is why Andrew and Varian were drawn to eachother. Eugene got a lot of sympathy food as a kid because of this. Adira is just confused. Eugene also wonders what the weird buzz he gets around certain people is. Hector will not care and chase you off regardless.
The brotherhood is basically the royal guard for the current monarch. They are trained from a young age to protect the current prince/princess who then becomes king.
If Varian and Eugene grew up in the dark kingdom, Varian would've most likely been apart of Eugene's brotherhood.
Quirin is the impulse control of Edmund's brotherhood.
Quirin: No Edmund, you can't try and harness to moonstone for a dare. Adira don't you dare drop kick that child. Hector don't you even think about running your rino through the main hall!
At some point (post canon), the Lunarians will return to the kingdom and rebuild. Then the group (Brotherhood, Cass, Raps, Eugene, Varian, Lance other relevant people) go on a search for the lost capital and find it. They release the curse and the Lunarians reclaim their capital.
The group also discover Old Saporia and adventure through the decrepid ruins. Due to the Sundrop being gone, the life that was once abundant in Saporia is now gone and withered.
This will cause Rapunzel and Cassandra to see just how much the conflicts over the Sundrop and Moonstone have affected the two kingdoms.