(Some light spoilers ahead for Season 1 of "Marvel's Iron Fist" and "The Defenders" on Netflix.)
A central part of the story of Netflix's "Iron Fist" Season 2 is the fight that led to Danny Rand (Finn Jones) becoming the Immortal Iron Fist, a highly trained martial artist who can channel power into his fist and punch through just about anything -- or anyone.
Danny faced off against his best friend and adopted brother, Davos (Sacha Dhawan), for the honor of becoming the Iron Fist. Both lived in K'un-Lun, a mystical city far removed from the rest of civilization. If you haven't watched the first season of "Iron Fist," however, you might be forgetting what exactly the deal is with K'un-Lun, or what's so weird about it.
When Danny shows up back in the New York in the first season of "Iron Fist," he's been missing for some 15 years. Presumed dead in a plane crash over the Himalayas, nobody he knows even believes he really is who he says he is. Where has he been all this time? According to Danny, he was living in a city called K'un-Lun.
Danny also says he's been serving the city as a warrior, and that his role as Iron Fist is to fight The Hand. That's the organization of evil ninjas that showed up in the second season of one of Netflix's other superhero shows, "Daredevil." The Hand serves as the main set of villains in "The Defenders," when Danny Rand teams up with Daredevil, Jessica Jones and Luke Cage and stop them once and for all. In Season 2, we see a few flashbacks of K'un-Lun, as well as Danny's adversarial relationship with Davos as the two compete to become the Iron Fist.
But what, exactly, is K'un-Lun? Danny's description of the place in "Iron Fist" is cryptic -- it's a capital city of Heaven, he says, and it only appears on Earth once every 15 years. That explains where he's been, and why it took him so long to return to New York.
The city is located in the real-world Kunlun mountain range that's found in China. Kunlun Mountain is also a prominent place in Chinese mythology, with the fictional city perhaps giving rise to those real-world myths, although Kunlun Mountain itself is mythological and not real. But K'un-Lun's similarities to a real place end there.
Danny's description may sound a bit weird to people not familiar with the mythology, but K'un-Lun has plenty of history in the Marvel comics from which "Iron Fist" is adapted. The mystical city is, in fact, located in another dimension. There are seven capital cities of heaven in total, and all of them phase in and out of the Earthly plane.
Like the realms seen in "Doctor Strange," another mystical entry into the Marvel Cinematic Universe, K'un-Lun is a place of magic. In fact, it's usually magic that's required to get people there. For Danny, it seems he was lucky to wind up in the city, since it only appears on Earth periodically.
K'un-Lun is the Home of the Immortals in the comics, as well as the Iron Fist, a power which passes from warrior to worthy warrior. It's also home to dragons. In fact, in Danny's comic origins, he gained the Iron Fist power by claiming defeating a dragon who uses battle to gauge the worth of each new warrior to wield it. Danny repeatedly has talked about fighting the dragon, but we still haven't seen what actually happened -- or whether "dragon" is a metaphor.
In any case, Danny won the fight against Davos to get the opportunity to fight the dragon, and managed to win that fight, too. He became the Iron Fist, but left K'un-Lun to return to New York. When he tried to return at the end of Season 1, Danny found the pathway to K'un-Lun littered with bodies -- and the way to the city closed.