The Nine Realms are worlds, nine distant planets, interconnected through the world tree, Yggdrasil. As of Thor: Ragnarok, following Asgard's destruction; only eight realms remain.
History
Thor
In 965 AD, the Frost Giants of Jotunheim began to advance and conquer the other Realms using the power of the Casket of Ancient Winters. They begin with Midgard, on the planet Earth which is the focal point of the nine realms and the easiest place to connect a stable wormhole in Midgard. Using a wormhole to travel there they were confronted by the Asgardians who were there to battle them and stop their progress. After an unknown length of time the Jotun were finally driven back to Jotunheim, then the Casket and an infant Frost Giant were taken back to Asgard and a truce is established with the Jotuns.
Hundreds of years later (in present day), Thor is taking his place as the new king of Asgard. The coronation ceremony is interupted by Frost Giants who are later revealed to have entered through secert passageways between the Realms and undetected by Heimdall. Thor and his friends then travel to Jotunheim and ignore the truce and battle the Frost Giants.
After Odin retrieves them, he banishes Thor to Midgard to learn humility and wisdom. After Loki learns of his true heritage and becomes king, he visits Thor on Earth. Later, the Warriors Three and Lady Sif disobeys Loki's command and go to retrieve Thor. After Loki sends the Destroyer after them and a battle ensues, the humans (most notably S.H.I.E.L.D.) are aware of the existance of technologically superior beings.
Loki has negatiated for a party of Frost Giants, including Laufey, to invade Asgard to slay Odin while he is in the Odinsleep. After the Frost Giants have all been killed, Loki tries to destroy Jotunheim using the Bifrost, but is stopped by Thor, which results in Loki falling into the abyss of space.
Thor: The Dark World
Five thousand years ago, in the time of the Convergence, the Dark Elves of Svartalfheim tried to plunge the universe back to darkness as it was before creation. They were stopped by the armies of Asgard and they were all assumed to be killed off. However, when thousands of years later (present day) a ship full of escapees lead by Malekith were reawakened after the Aether was uncovered, they traveled to Asgard to retrieve it.
After the destruction of the Bifrost, enemies of Asgard such as Marauders, Frost Giants, Trolls, and Badoon have taken advantage and wreaked havoc across the Nine Realms. After the Bifrost is repaired, the armies of Asgard were sent to reestablish order. After a final battle in Vanaheim, Thor visits Jane on Midgard, where he learns she has found portals between worlds and has acquired the Aether.
Jane is taken back to Asgard and soon after, the Dark Elves attack. They do not manage to retrieve the Aether but do heavy damage to the Realm and its inhabitants. Thor and his friends release Loki from custody and embark on a dangerous mission. Loki leads them to Svartalfheim, home of the Dark Elves, through a secert portal.
Once there, their plan goes wrong and after sheltering in a cave, Thor and Jane find a portal back to Earth. They reunite with Erik Selvig, who has been studying the Convergence, and form a plan to intercept Malekith in Greenwich. When Malekith tries to revert all the realms back to darkness, he and Thor battle across the Realms through portals and Thor eventually crushes him in Svartalfheim.
After traveling back to Asgard and taking to Odin, who is really Loki in disguise, Thor decides to stay on Earth.
Agents of S.H.I.E.L.D.
"The Well"
After the battle in Greenwich, Coulson's team is called to clean up the area. An Asgardian artifact is found in a Norwegian forest and revealed to be the Berserker Staff . In the 17th century, Asgardians were recruited to join the army to go to Midgard. A stone mason, tired with his life and curious to explore the Realms, signed up. After the battle was over and the Asgardians called back, the stone mason stayed behind on Earth with the Staff. He broke it in three pieces and hid them across Europe.
The Warrior Who Stayed was later discovered by Coulson and revealed as Dr. Elliot Randolph and recruited to help the recollection of the Berserker Staff. However, a group of Norse paganists assembled it first and touching it experienced its "dark magic" and felt rage and gained heightened physical abilities. After being defeated, Randolph has decided to relocate and keep his identity a secret.
"Repairs"
After an accident in a particle accelerator lab, the maintenence man Tobias Ford was presumed dead. However, he was later seen as a ghost of sorts with the power to teleport. His power is revealed to come from the use of portals caused by the Convergence; he was teleporting back and forth from another realm, presumely Hel. Everytime he teleported, his ability became diminished until eventually, he was convinced to stay away from Midgard.
"T.R.A.C.K.S."
Asgard is mention when a train seems to have teleported.
Realms
- Alfheim - Home of the Ljósálfar or Light Elves.
- Asgard - Home of the Asgardians. It was ruled by Odin Borson.
- Vanaheim - Home of the Vanir who are the sister race of the Asgardians.
- Jotunheim - Home of the Jotun or Frost Giants. It was ruled by Laufey.
- Midgard - Home of the Humans.
- Nidavellir - Home of the Dvergr or Dwarves.
- Svartalfheim - Home of the Svartálfar or Dark Elves. It was ruled by Malekith.
- Nifflheim - Home of the dishonored dead which is distinct but closely connected to Hel.
- Muspelheim - Home of the Muspeli or Fire Demons. It was ruled by Surtur.
Other Realms
- Hel - Realm of the dead. It was Ruled by Hela.
- Valhalla - Home of the honored dead and is distinct but closely connected to Asgard.
Appearances
- Marvel Cinematic Universe (2 films)
- Thor (First appearance)
- Thor: The Dark World
Trivia
- In the comics, the Nine Realms are parallel dimensions. However, in the Marvel Cinematic Universe, the Nine Realms are a group of distant planets.