Netflix's upcoming Shadow & Bone series centers on the treacherous Shadow Fold, including explaining what, exactly, the Shadow Fold is. Netflix's adaptation of Leigh Bardugo's bestselling and widely beloved book series is set in an earthy fantasy world that mirrors our own, where war sits side-by-side with prosperity and magic and science work hand-in-hand.

In particular, the nation at war - and where the majority of Shadow & Bone's action takes place - is the kingdom of Ravka, a country torn apart by civil war after the Darkling attempted to overthrow the country's ineffective and feckless ruler, Alexander III. It's a land of magic users, the Grisha, and non-magical people, sometimes referred to as otkazat'sya - "the abandoned." Normal humans make up Ravka's First Army while the Grisha make up the Second Army, and even with two armies, Ravka is stretched to is breaking point, being torn apart from inside by civil conflict and threatened from outside by other nations that would take advantage of its turmoil.

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Worse, Ravka is not just metaphorically torn apart, but also literally. The Shadow Fold, also known as the Unsea, suddenly appeared one day in Ravka, splitting the country in two, and has been steadily growing ever since. Its presence threatens the entire kingdom, but few know how to survive it and no one knows how to stop it. The Shadow Fold is practically a character in its own right in the books, and the new 2021 Netflix show looks to bring it to life. Here's why it's so important - and so terrifying.

What Is The Shadow Fold?

The Shadow Fold is a tear of pure darkness that splits Ravka in two. It's also colloquially known as the Unsea as, rather than water, it's the absence of it, of everything. It's as though an inland sea of pitch-black shadow suddenly spilled out of a crack in the ground and spread over the land. No one knows how it began and no one knows how to destroy it, or even if it can be destroyed, and its existence has slowly been draining the life from Ravka.

The problem is that, due to its location, the Shadow Fold cuts off the majority of Ravka, including its capital, Os Alta, from the rest of Ravka. Worse, it cuts right across the Vy, the giant, main road that once connected Os Alta and most of Ravka to Ravka's port cities and trade routes on the coast of the True Sea to the West. Without the ability to safely and regularly navigate back and forth across the country, it's hard to get goods and supplies to the majority of the country cut off from its normal trade routes and Ravka is slowly strangling because of it.

Why Is The Shadow Fold So Dangerous?

Shadow and Bone Volcra

Crossing a sea of nearly impenetrable darkness is hard enough, but it can be accomplished easily enough with lights and skilled navigators - or so one would think. The problem is that lights simply don't work very well in the Shadow Fold; it's not just the normal darkness of night falling, but an almost sentient, writhing mass of darkness that resists light, seeming to swallow it up immediately.

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Even more dangerous than the darkness itself, however, are the ravenous monsters that lurk within the Fold, known as volcra. The volcra are hideous creatures that feed upon human flesh. They're terrifying in appearance, with large, leathery wings razor-sharp, dirty claws that rend people to shreds and infect the ones they don't kill, and rows of jagged teeth. They've lived in the Shadow Fold for so long that they've gone blind from lack of light, but, much like sharks in water, they can smell human blood miles distant. The volcra aren't the only monstrous denizens of the Fold, but they're by far the worst creatures lurking in the dark.

Understandably, crossing the Shadow Fold is an extremely dangerous affair and numerous precautions are taken to ensure a relatively safe passage and that everyone crosses through and back alive. Special boats called sandskiffs are used to traverse the Fold. A cross between a ship and a sled, sandskiffs are purposely fashioned as large, shallow sleds with a single mast, meant to be propelled by a Squaller - a Grisha who can harness the air and wind - to glide the skiff soundlessly over the sand.

Can The Shadow Fold Be Destroyed?

Shadow and Bone New Poster

Most Ravkans have resigned themselves to the Fold's stranglehold on their country and its inexorable expansion day by day. No one quite knows where the Shadow Fold came from, only that it has to be magical in nature as it was not there and then, overnight, it was. Nor does anyone know what it is other than living darkness. Hence, no one knows how to destroy it, or even if it can be destroyed.

That's where Alina Starkov comes in. When the book series opens, she's a sickly, frail teenager growing up in an orphanage in Keramzin under the care of Duke Keramsov. Though weak and often sick, Alina is content to be at the side of her best friend since childhood, Malyen Oretzev. When they're both drafted into the Ravkan First Army and sent as part of a mission to cross the Shadow Fold, the crossing goes awry. Their sandskiff is attacked by volcra and Mal near fatally injured. In the panic and confusion and chaos, a sudden burst of pure light explodes from the skiff, obliterating the volcra and saving the survivors on board; this moment is dramatically portrayed in the first trailer for the upcoming Netflix TV show.

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As it's discovered, the burst of sunlight was from Alina, and her true nature is soon revealed: She's a Sun Summoner, the rarest of Grisha. So rare, in fact, that Alina is the only one of her kind currently alive. Alina's newly-discovered magical powers grant her the ability to summon and create pure sunlight and heat in massive quantities. As she uses her magic, she grows in health, realizing that she was so sickly because she'd unknowingly repressed the magic she possessed without being aware. It quickly becomes clear that even though her magic was latent for far longer than the age at which most Grisha start to exhibit magical inclinations, Alina is incredibly powerful. It puts a heavy burden on her for Alina, and Alina alone, is the only person in all of Ravka who has the magic to destroy the Shadow Fold. This fight to destroy the Fold and save Ravka will presumably be the big focus of Shadow & Bone season 1, judging from the trailer:

It will be fascinating to see how both the Shadow Fold and Alina's powers are depicted in the Netflix series.

Netflix's Shadow & Bone premieres on Friday, April 23.

More: What Is Shadow & Bone? Books, Characters & Netflix Show Explained