To approximate the atmosphere of the book (which lacks dialogue and doesn't give the protagonist a name and I feel has a kind of Temple Grandin thinking in pictures vibe) it would be in the format of an old silent film, without dialogue, mostly just images and music, with some use of cue cards to establish context. Cue cards are in this summary rendered as ALL CAPS.
I think this would be appropriate music for it:
I'd develop Naani's side of the story more, make it a symmetrical companion narrative to X's, and to that end my depiction of the fall of the Lesser Redoubt would deviate substantially from the information we get about it in the novel. I took some inspiration from Nigel Atkinson's A Mouse In the Walls of the Lesser Redoubt, but my version wouldn't exactly follow that narrative that either.
Opening shot would be in space, showing stars, the sun, the moon, and the Earth, almost unrecognizable. The sun has become much dimmer, like a white dwarf, and the Earth and moon are little more than disks of blackness against the stars. The viewpoint begins to move toward the dark Earth.
MILLIONS OF YEARS IN THE FUTURE. THE SUN IS DYING, AND HAS ALMOST CEASED TO GIVE OUT VISIBLE LIGHT.
The viewpoint moves in toward the Earth. Passes briefly over the ruined highlands, with their frozen oceans and glaciers and, poking out here and there, the crumbling remains of huge buildings, just barely visible in the dim sunlight, which is like present-day moonlight. It comes to something like a huge rift valley, the bottom of which is an eerie landscape dimly lit by volcanic activity and bio-luminescence - except, there is also a brightly lit and unmistakably artificial three-sided pyramid down there, which the viewpoint quickly approaches and circles like a bird.
THE LESSER REDOUBT, OR THREE-CORNERED WORLD. PERHAPS THE LAST HUMAN SETTLEMENT.
The pyramid sits atop a large mesa, and around the mesa there is a glowing circle. The viewpoint comes closer. We see that the circle is something like a large glowing conduit, and inside its inner perimeter there is something like a wall, patrolled by armored and armored human figures - I'm thinking something like Matt Isitt's design for the weapons and armor.
THE AIR CLOG AND THE PERIMETER WALL AND THE GUARDIANS KEEP OUT THE PREDATORS OF THE NIGHT LAND.
BUT THE DIREST ENEMIES ARE COLD AND DARKNESS, AND THE WALLS AND LAMPS AND HEATERS OF THE REDOUBT KEEP THEM OUT.
The viewpoint moves into the pyramid. Now we're seeing interior scenes; public spaces filled with people, huge machinery being worked by technicians, gardeners tending plants in huge artificially lit gardens. I think I'd take inspiration from Mexican Art Deco/Nouveau architecture for the Lesser Redoubt's interior spaces, so they'd look a little like this:
But a lot of the architecture and machinery is noticeably run down, and everyone looks similar, like they're all related.
Shots of technicians tending huge machines.
THE LIGHTS AND HEATERS AND THE AIR CLOG AND EVERYTHING ELSE IN THE REDOUBT IS POWERED BY ENERGY TAPPED FROM THE EARTH CURRENT.
A PERFECTLY ISOLATED POPULATION, THE REDOUBT IS SLOWLY DYING. SKILLS ARE LOST WHEN PEOPLE DIE BEFORE THEY CAN PASS THEM ON. SO ARE GENES; ITS PEOPLE ARE BECOMING DANGEROUSLY INBRED. MACHINERY DECAYS BECAUSE THERE ARE NOT ENOUGH PEOPLE TO DO ALL THE WORK THAT NEEDS TO BE DONE. ONCE LOST, KNOWLEDGE IS NOT EASILY REDISCOVERED, AS IMMEDIATELY NECESSARY WORK MUST BE PRIORITIZED OVER SCIENTIFIC INVESTIGATION.
THE REDOUBT HAS DEFIED THE ULTIMATE NIGHT FOR A MILLION YEARS. BUT ITS PEOPLE KNOW THAT IT IS SLOWLY DYING, AS THE GREAT WORLD DID BEFORE IT.
THEIR HISTORY BOOKS TELL OF ANOTHER HUMAN SETTLEMENT, A GREATER REDOUBT. BUT THE LAST CONTACT WAS LONG AGO. SHIFTS IN THE LAND HAVE MADE THE LONG ROAD TO THE GREATER REDOUBT HARD. SIGNAL CABLES FAILED AND WERE NOT REPAIRED LONG AGO. RADIO AND AIR TRAVEL ARE NOW LOST ARTS.
EVERY FEW SLEEP-WAKE CYCLES, THE PEOPLE TRY TO CONTACT THE GREATER REDOUBT THROUGH THE MASTER WORD.
The people stop what they are doing. Delicate-looking, complex, golden, glowing things, reminiscent of Arabic calligraphy float up from their heads, float through the air, and gradually disperse and fade. The viewpoint follows some of these glowing floating things as they drift out of the Redoubt and over the Night Land. Some drift over the Night Land until they fade to nothing. Some drift up the kilometers-high cliffs at the edge of the deep lowland the Redoubt is in and dissipate in the highlands where glaciers slowly settle over the ruins of the human civilization of the Bright Days. Some continue to drift across the valley floor, until... They come within sight of an immense brightly lit four-sided pyramid of literally mountainous dimensions, an instantly recognizable bigger sibling of the Lesser Redoubt: the Greater Redoubt! Some of the glowing floating things fade and dissipate before they can reach the bright four-sided pyramid. A few make it inside, and float through the bodies of people in the Greater Redoubt... but the people they float through show no reaction, no awareness of the floaters.
INBREEDING HAS WEAKENED THE MASTER WORD. THE PEOPLE OF THE LESSER REDOUBT CAN NO LONGER TRANSMIT IT STRONGLY ENOUGH FOR THE PEOPLE IN THE GREATER REDOUBT TO HEAR THEM.
THERE IS A LEGEND THAT IF SOMEONE CALLS OUT THE MASTER WORD IN A MOMENT OF GREAT NEED AND PAIN, ONE WHO LOVED THEM IN A PAST LIFE WILL BE ABLE TO HEAR, EVEN IF OTHERS CANNOT...
THE END COMES SLOWLY, THEN QUICKLY...
The huge machines that tap the Earth Current. An alarm goes off. Technicians frantically making adjustments. A great shaking like an earthquake, and an explosion.
Exterior shot of the three-sided pyramid. The pyramid goes dark.
THE MACHINES THAT TAP THE EARTH CURRENT HAVE FAILED.
Interior shots. People going about their business in the Redoubt. People working in the gardens. Each one ends with the lights going out.
The armored guardians outside the Redoubt are plunged into almost-darkness as the glowing circle and the lights of the Redoubt go out.
IT HAS HAPPENED BEFORE. THERE ARE EMERGENCY BATTERIES AND GENERATORS. REPAIRS ARE ATTEMPTED.
The pyramid lights up again, though less brightly.
Engineers in consternation before the huge, ancient machines that tap the Earth Current. Trying to make repairs.
THE ENERGY AND FUEL IN THE EMERGENCY BATTERIES AND GENERATORS BEGIN TO RUN LOW.
The glowing circle at the base of the mesa goes dark. The armored guardians look at each other in trepidation.
THE BLOOD OF THE GUARDIANS WILL BE SPENT SO THE CIVILIANS CAN CONTINUE TO HAVE LIGHT, WARMTH, AND FOOD.
The armored guardians outside the Redoubt in battle. Their enemies are things like huge black wolves with glowing eyes.
The pyramid goes dark again. More interior shots, each ending with the lights going out.
THE EMERGENCY GENERATORS HAVE RUN OUT OF FUEL.
More scenes of the engineers trying to fix the machines that tap the Earth Current. One of them collapsing, crying, being comforted by others.
THIS TIME THE MACHINES ARE BROKEN BEYOND REPAIR.
A great exhalation of the glowing calligraphy-like floaters of the Master Word from the Lesser Redoubt. We see that some make it to the Greater Redoubt, but the people of the Greater Redoubt cannot perceive them.
A grim meeting of the city's leaders. The room is dim, lit only by emergency lights.
"THE GREATER REDOUBT MAY YET SURVIVE. THEY ARE OUR ONLY HOPE."
"WE SHOULD SEND ONE OF THE GUARDIANS, SOMEONE STRONG, TO TRAVEL THE OLD LONG ROAD, TO SEE IF THE PEOPLE OF THE GREATER REDOUBT YET LIVE, AND, IF THEY DO, TO TELL THEM WHAT WE ARE SUFFERING."
"EVEN IF THEY YET LIVE, AND WOULD AND COULD HELP, IT WOULD TAKE TOO LONG."
"WHAT ALTERNATIVE IS THERE?"
"WE SHOULD ALL GO. ALL OF US. ALL THE PEOPLE."
"WE SHOULD PUT IT TO A VOTE."
A great assembly, in an auditorium dimly lit by emergency lighting. Ballots being taken.
"THE THREE CORNERED WORLD IS DYING, AS THE GREAT WORLD DID BEFORE IT. SOON THERE WILL BE NOTHING LEFT FOR US HERE BUT COLD AND DARKNESS AND GRIEF. SO WE WILL ALL GO. ALL OF US. ALL THE PEOPLE. IN CARAVAN, DOWN THE OLD LONG ROAD ACROSS THE NIGHT LAND. IT IS OUR BEST HOPE FOR SURVIVAL."
"IT IS A SLIM AND DESPERATE HOPE, BUT WE DO NOT HAVE A BETTER ONE."
Preparations are made. Supplies are put into containers and the containers are put onto improvised carts and sleds and trailers and into backpacks. Bedding and light clothing designed for inside the climate-controlled Redoubt are torn apart and sown back together into improvised cold-weather clothing. Chains are put around the wheels of construction machinery so they won't slide in snow while being used as improvised tractors for trailers. The Great Vehicles are brought out from the Hall of Dead Generations.
A huge gate opens at the base of the Redoubt and the column of refugees set forth. They have some vehicles to carry people and supplies, but nowhere near enough to carry everything and everyone; the majority of the people have to walk. The people are bundled up against the cold of the Night Land, and their breath steams in the cold air. Armed and armored guardians walk at the front, back, and sides of the column, to protect the civilians from the predators of the Night Land. They set out along the old long road.
They travel (various cues are used to suggest considerable time passing).
They come to a place where the land has risen. The road now ends in a place of rough, jumbled rocks. With difficulty, a person can walk across this terrain, but it is impassible to the vehicles.
Scouts are sent out, but when they return they report they have found no way around the obstacle for considerable distance.
The vehicles must be abandoned. Supplies and equipment are unloaded from them, to from here be carried on people's backs. Improvised stretchers are made, and people who cannot walk are helped off the vehicles and into the improvised stretchers, so they can be carried. Much supplies and equipment are abandoned with the vehicles; too bulky and heavy for unassisted humans to carry.
The people slowly make their way across a stretch of rough, rocky terrain. Again, cues are used to indicate a significant time passing; if there were still days, it would be days. Eventually they come to a place where the road is relatively intact again.
The people continue walking. More indicators of time passing. The road begins to pass along the floor of a long gorge.
All through the journey, little golden calligraphy floaters of the Master Word go up from the traveling column of humans and disperse into the night. Sometimes they go up in ones, twos, threes, fives, sometimes there is a great simultaneous exhalation of very many of them.
Somebody collapses. Others help them back up. On the high ground above the road, one of the creatures like huge black wolves is watching. We see its thoughts. It imagines itself attacking the person who collapsed, dragging them off into the darkness and feeding on their flesh. Then a second thought: itself trying to do that, being slain by one of the armored guardians. Then a third thought; many creatures like it attacking the refugee column simultaneously, some are slain but they overwhelm the defenders and the ones that are not slain feast.
We see the three scenarios playing out in parallel in three thought bubbles above its head. Then the thought bubbles have something like glowing threads around them, as if they're a bundle bound together with string. This is indeed the visual metaphor being developed; this bundle begins to float away from the creature. The creature generates another bundle like it, which begins to drift away too, and then another. The viewpoint follows some of the bundles as they float away across the Night Land, until they encounter another of the black wolf-like predators and enter it and disappear into it, at which point the creatures instantly raise their heads alertly (one of them is sleeping, and springs to its feet when the bundle disappears into it), look to a point on the horizon with intent, and begin to walk in that direction.
A stylized depiction: the refugee column walking along the road, seen from far above, and a great number of the wolf-like predators are following it and converging on it from many directions.
One of the predators dislodges a stone from the lip of the gorge. It falls down the sloping sides of the gorge. Down in the gorge, somebody looks up and sees many shining eyes looking down from the darkness on the top of the gorge. Spotlights and flashlights are turned on the top of the gorge, revealing many of the wolf-like predators walking atop it and some of them beginning to make their way down the slopes.
A roiling agitation of the moving crowd. Fear. Screaming. A mother grabbing a toddler off the ground to hold them close and carry them.
At the front and back and sides of the column, armored Guardians spin up their diskoi (the circular saw pole-arm this person is holding) and hold them tightly.
A man in Guardian armor with his helmet off and some older people conferring tensely, eying the advancing predators nervously.
"SHOULD THE PEOPLE DISPERSE?"
"THE TERRAIN HERE TRAPS US. THEY ARE CUNNING CREATURES."
"THERE IS A REASON WE CHOSE TO TRAVEL TOGETHER. ALONE OR IN SMALL GROUPS WE CAN BE PICKED OFF ONE BY ONE. THE GUARDIANS CAN DEFEND THE PEOPLE BETTER IF THE PEOPLE ARE IN A MASS. AND WE WILL BE MORE LIKELY TO SURVIVE THE JOURNEY TOGETHER."
Fear is starting to curdle into panic. People are starting to run. Somebody falls and is stepped on. The walking column is in danger of becoming a stampede.
An older woman still wearing the remnants of some fancy clothing along with her improvised insulating jacket and leggings and the blanket she's wrapped around herself to keep warm climbs onto the shoulders of the leader of the Guardians. The leader of the Guardians is walking backwards and near the head of the column. He and the woman on his shoulders are holding recognizably megaphone-like devices, and with her other hand she holds some jeweled badge of office high in the air.
"THE GUARDIANS WILL DEFEND THE PEOPLE!"
"STAY TOGETHER! WE ARE STRONGER TOGETHER! WE WILL GET THROUGH THIS TOGETHER!"
"NIGHT-HOUNDS CAN READ SIGNS OF HUMAN FEAR AND INTERPRET THEM AS SIGNS OF WEAKNESS! DO NOT LET THEM SEE YOU ARE AFRAID! WALK, DO NOT RUN!"
"KEEP YOUR CHILDREN CLOSE!"
The people take up the ... suggestion? Command? The column resumes a relatively orderly walking progress. The armored Guardians walk at the front and rear and sides of the column, their spinning diskoi held ready.
A steady stream of little calligraphy golden floaters of the Master Word goes up from the column of humans and drifts away into the ultimate night; sometimes they go up in ones or twos, once there is a huge exhalation of many all at once.
For a while there is a tense stand-off. The night-hounds are much smarter than their ecological equivalents from the sunlit times, and they can use their thinking-in-pictures distant cousin of the Master Word to coordinate much better, but they are still much less social and much more selfish than humans, and here they've encountered a collective action problem. They're smart enough to realize the vanguard of the attack can be expected to take heavy casualties, and none of them want to be in it. Every hound hesitates, hoping others of its kind will take the initiative to attack and thus accept being in the vanguard.
In the end, a sort of hierarchy of desperation begins to shake out among them. The hungrier, leaner, sicklier, more desperate hounds are tempted toward the column of humans, while the better fed and therefore less desperate hang farther back.
In the end, they hit upon a solution many human armies of the sunlit times used: file closers. The better-fed and stronger hounds begin to attack the hungrier, leaner, weaker ones that have ventured closer to the humans and feed on them cannibalistically. Trapped between the jaws of their stronger cousins and the diskoi of the Guardians, the weaker, leaner hounds begin to choose the latter.
"GUARDIANS, LET NO ENEMY THROUGH YOUR LINES, NO MATTER WHAT!"
The Guardians defend the people bravely. To the last Guardian. There are too many night-hounds, and this battle is the last stand and Ragnarok of the people of the Lesser Redoubt, and in the end the night-hounds that don't die by the diskoi of the Guardians or the improvised weapons of desperate civilians get the feast they came for.
All through the battle, little floaters of the Master Word continue to go up and disperse into the night.
In the last, bloodiest, most desperate phase of the battle, as the battle shades with queasy ambiguity into the victors' enjoyment of their gruesome reward, a single young woman manages to survive by hiding a crack in the earth.
After the battle, she wanders around the road with a flashlight. Its beam illuminates nothing but ancient road surface, barren earth, blood, gore, and corpses. She finds and weeps beside the corpses of her mother and father. She curls up on the side of the road, rocking and covering her ears with her hands, and a Master Word floater rises up from her.
The viewpoint follows the floater into the night.
It crosses the Night Land, to the still shining pyramid of the Greater Redoubt. It floats into the Greater Redoubt. Sometimes it floats through people, even through their heads, but they do not show any reaction. Then it disappears into a young man sitting at a table reading something, and he suddenly stands up, visibly shocked.
Scenes of the young man talking and arguing with other people.
AFTER SO LONG WITHOUT CONTACT WITH THE LESSER REDOUBT, THE OTHERS ARE SKEPTICAL.
IF THE VISION IS TRUE, THEY WANT TO HELP THEIR DISTANT COUSINS, BUT...
THE OLD ROAD IS LONG AND THE NIGHT LAND IS DANGEROUS. THOSE WHO VENTURE PAST THE AIR CLOG MAY NOT RETURN.
THEY ARE RELUCTANT TO RISK THE WASTE OF PRECIOUS RESOURCES AND LIVES THAT MIGHT HAPPEN IF THEY ACT ON THE VISION AND IT IS AN ILLUSION.
HE OFFERS TO GO OUT INTO THE NIGHT LAND ALONE, TO SEEK THE ONE WHO HAS CALLED OUT TO HIM.
We see the young man, now in armor and with a diskos, being given a farewell ceremony at a gate of the Last Redoubt and leaving the Last Redoubt (he doesn't put his helmet on until after he walks out the door, so we know it's him).
After this we get a fairly faithful to the novel depiction of the young man's crossing of the Night Land; the battle against the Grey Man, coming to a place where the old road ends in land jumbled by geologic activity after its construction, the journey across the country of darkness...
I think a good way of rendering the journey across the country of complete darkness would be, like... At first we see the young man making his way across that country with the help of an electric light. He periodically checks an indicator, and we can see that the light's batteries are running low (the indicator should be something intuitive). Maybe have a cue card saying the battery is running low and he's going to risk continuing in darkness. Up to now in this sequence the things illuminated by the light have been visible. When he turns the light off, the only things visible are now the man, and a very small area of things he's touching or has explored through touch. So, when he's walking, we see him in almost a black void (maybe there are stars above, and a line of a horizon visible by where they end), with tiny circles around his feet when he puts them down (so, we may occasionally see a rock by his boot or something). Then he trips and takes a bad fall, as in the novel. His body strikes something sharp, and the edge of it becomes visible in the void. After recovering, he explores the sharp thing by touch, and we see a big, sharp rock. He explores his armor where he hit it by touch, and we see a scratch on the armor. After that, he dares not walk, but continues the journey crawling on his hands and knees. Again, he appears to be in an almost in a black void, with only things he's touching and tiny circles of whatever he's recently explored by touch illuminated. Eventually he comes to an area where there's some dim lighting by volcanic activity and bio-luminescence and we're able to see a landscape again and he starts to walk normally normally again. Maybe have a cue card at this point something like
... From there the film would continue to follow the novel fairly faithfully; the journey along the underground river (and avoiding being noticed by the big slug creature), the journey through the dark forest...
Interspersed with the young man's heroic journey, we get scenes of the young woman survivor trying to survive in the Night Land, occasionally finding and eating some edible wild plants or fungi, but becoming noticeably weaker and visibly thinner and thinner, eventually to the point of emaciation.
Every once in a while a Master Word floater goes up from the young woman or the young man. Sometimes they dissipate. Sometimes one sent out by the young woman reaches the young man or vice versa, and they visibly react to it (sometimes with joy, sometimes the young man will react with visible increase of urgency in his travels).
At one point we see the young woman, starving, thin, staring at a small animal.
THE REDOUBTS MUST BE EFFICIENT. THE FOOD OF THE REDOUBTS IS VEGETARIAN. LIVESTOCK WOULD BE INEFFICIENT.
THE CULTURAL MEMORY OF HUMAN CARNIVORY HAS BEEN LOST.
IT DOES NOT OCCUR TO HER THAT SHE MIGHT HUNT AND EAT AN ANIMAL.
The animal ambles away from her, untroubled.
... The young man finds the old road again, travels down it to the empty shell of the Lesser Redoubt, meets the young woman nearby...
That's about the point in the novel I've gotten up to so far, I've been doing other stuff, but based on the Wikipedia summary I guess from there the film could follow the plot of the rest of the novel relatively faithfully.