When I was a kid, I had a keen interest in mythology. Being Norwegian, the mythology closest to my heart was norse mythology. I lapped up any information I could get my hands on concerning Thor, Loki, Balder, Hod and the rest of the inhabitants of Asgard. As I grew up, I branched out to Celtic, Christian, Hindu, Greek and Japanese mythology, but what really got my interest was getting introduced to the heroes of modern mythology: superheroes. Trying to dive into the X-men lore, the history of Spider-man, the saga of the Fantastic Four, it all felt familiar, and I loved it. But it was when I got reacquainted with Thor of Asgard, now a superhero operating in the United States, that really interesting things started happening.
In the Marvel phase one movie Thor, we were introduced to the concept of the nine realms of Yggdrasil, the world tree. We also got to see three of these realms; Asgard, Midgard and Jotunheim, and in Thor: the dark world, we'll be introduced to Svartalfheim as well. But why stop there? Here's a quick introduction to all the nine realms of Yggdrasil.
The nine realms are separated into three categories. The upper realms, the middle realms, and the lower realms.
Asgard, home of the Aesir. This is the home of the norse gods, an advanced race of warriors who've taken it upon themselves to protect the peace amongst the nine realms. They travel the realms with the help of Bifrost, the rainbow bridge. Odin is the king of Asgard.
In the comics, Asgard is closely related to Vallhalla, where valiant warriors who died in battle have their home.
Vanaheim, home of the Vanir.
The Vanir are related to the Aesir, but deals more in magic than
their cousins, and there's been several conflicts between the two
worlds. Still, some Vanir has decided to move to Asgard, Frigga, who
tutored Odin in the secrets of magic, her brother Frey and their
Alfheim, home of the Light Elves. These guys seem pretty doctile and haven't had all that much screentime in comics. They're elves, of the classic pointy eared kind. Still, there is one interesting tidbit. The climate in alfheim has been shown to be of the extreme variety, ranging from the greenest spring to cold and snowy winter. Also, either the elves change color from pinkish to light blue in sync with the climate, or there are two varieties.
The middle realms
Nidavellir, the dwarven lands. Dwarves live here. It's the industrial realm, where things like magic hammers are made. Dwarves are not exactly a friendly people, but if you need something made, and have an insane amount of gold to match their price, they're your best choice.
Midgard, the realm of humans. Yes... it's earth. And what's worse, we're apparently in the bloody middle of all these dysfunctional worlds, so a warring horde will usually have to go through us to wherever they're going.
Jotunheim, Land of giants. They're big, live between rocks and hard places, and have anger management issues. In Thor, we got introduced to the frost giants little piece of heaven, but there's also a place for rock giants and storm giants in this realm. Loki is, despite his lack of height, a member of the frost giant-variety.
Svartalfheim, home of dark elves. These guys do not play well with others. They live in castles, forest cities and underground caverns, are blueskinned, and dream of conquest. They live longer than humans, and are physically better than us at just about anything, though still weaker than Aesir. In addition, they are very proficient with magic. Their one big weakness is iron. The otherwise basic metal can disrupt their spells and tear through even their hardest armor. Svartalfheim is the lowest of the middle realms, and is occationally considered amongst the lower realms.
The lower realms
Hel/Niffelheim. While separate, these two realms count as one, due to their strong connection and close proximity to each other. Hel is ruled by Hela, goddess of the dead, and is where the common folk who didn't die honorably in battle eventually ends up. It's not a nice place to be, but if you're not a warrior type, it's still better than the alternative. Niffelheim is the alternative. A cold frozen wasteland for the dishonored dead.
And at last Muspelheim, land of Surtur. Surtur is the king of the fire giants, and Muspelheim is his realm. It's a giant firepit where only he and his fire giants and demons reside. He hates Odin with a passion, and would love nothing more than to spread his fires to Asgard.
With the “magic is science, and gods are aliens”-approach of the movies, there's likely to be a couple of major differences here. Valhalla might be a place where the spirits of dead warriors resides, but it's doubtful the warriors in question are from anywhere but Asgard and Vanaheim. As for Hel/Niffelheim, I imagine that if any of them are mentioned or shown, it will be the frozen wasteland. There will be undead creatures there, but who and how they got there will either be unexplored, or there will be some kind of explanation regarding an ancient battlefield or curse or something. If we are ever to be introduced to any of the other realms besides Midgard, Asgard, Jotunheim and Svartalfheim, my best bet would be Vanaheim or Muspelheim. Both have beings or races that might be inclined to travel to earth to dig up some artifact or another that will help them conquer Asgard, and both have potential for some really stunning visuals.
If you wish to elaborate on any of these, or if you have suggestions for other worlds, realms or alien races you'd like me to explore, take your suggestions to the comments below.