Before he became the first White Walker, the Night King was a First Man that was captured by the Children of the Forest, Leaf among them. Leaf pressed a dragonglass dagger into his chest, causing his eyes to turn blue and turning him into the first of the White Walkers. Thousands of years later, Leaf tells Bran Stark that her people created the White Walkers to defend themselves when Westeros was invaded by the First Men, who were cutting their sacred trees and slaughtering the Children of the Forest.
The Night King first appears in a vision that Bran Stark has, when Bran communes with a Weirwood Heart tree. He experiences a flood of images from the past, present, and future, many of which he was not physically present for. He does not comprehend what all of these images are. In retrospect, one of them is an image of the Night King picking up the last of Craster's sons off an ice altar.
After Rast places Craster's final son on the ground in the Haunted Forest, a White Walker riding an undead horse approaches and takes the baby, carrying it towards a shattered mountain in the Lands of Always Winter.
Once inside, the White Walker approaches an icy altar ringed by large icy spikes and places the baby upon the altar. In the distance, a group of thirteen black-garbed White Walkers are revealed to be viewing the proceedings from afar. One of them breaks from the middle of their number and approaches the altar, stopping to regard the human child for a moment before gently gathering him in its arms. The baby immediately calms, staring into the face of the Night King, who has a crown of horns jutting from his head. He places his index finger upon the baby's cheek, causing the child's eyes to turn icy blue and its skin to grow pale: most likely the fate of Craster's other sons.
The Night King appears again when Jon Snow and Tormund Giantsbane are coordinating the evacuation of Hardhome. He appears to be leading the attack, or is at least observing it with some other White Walkers, and watches from one of the cliffs above the town as Jon slays one of his lieutenants with Longclaw. The Night King looks on this turn of events with mild interest, as he didn't think anybody would put up any sort of meaningful resistance. As Jon, Tormund and the remaining defenders leave on the boat, the Night King appears on the dock and locks eyes with Jon. With a mere raise of his arms, the Night King raises the entirety of Hardhome's erstwhile defenders as wights, and keeps his gaze upon Jon as the boat slips away.
The Night King appears in one of Bran Stark's visions, where he witnesses him as a human being transformed into a White Walker by the Children of the Forest, notably among them Leaf. She explains after his vision that they had to do it because they were at war with the First Men.
Later, Bran makes the risky decision to experience a vision on his own, and finds himself looking at an army of wights and the Night King and other White Walkers at the back. Bran walks closer and is shocked to see that the Night King notices his presence. Bran wakes up screaming after the Night King suddenly appears right next to him and grabs his arm.
The Three-Eyed Raven says that the Night King touched him, which Bran confirms when a blue hand-mark is seen on his forearm. The Three-Eyed Raven says because of that mark, the Night King now knows exactly where they are, and the cave cannot protect them any more.
The Night King and his army quickly travel to the cave, preceded by their aura of cold, a signal to the Children of the Forest and Meera Reed. The Children of the Forest prepare their scarce defenses outside of the cave, but are quickly overrun and they are forced to retreat back to the tunnels. The Night King sends his army of wights to attack the tunnels, and they kill most of the Children and Summer, though Bran, Meera Reed and Hodor manage to escape the cavern through a back tunnel. One of the other White Walkers is killed by a dragonglass-tipped spear thrown by Meera. The Night King enters the cavern, and glares at the warging Three-Eyed Raven before he kills him.
The Night King then sends the rest of his wights to kill Leaf, and eventually, Hodor, who attempts to stop the wights from going further by sacrificing himself to hold the door.
While Meera drags Bran through the snow, Bran sees visions of the Night King converting Craster's last son into a White Walker, leading a massive attack on Hardhome, and branding him with his mark.
In the aftermath of his victory at Winterfell and the confirmation from the Citadel that winter has come through white ravens, Jon Snow tells the gathered lords of the North and the Vale who wish to return home following Ramsay Bolton's defeat to prepare for winter. He warns that the war is not over after Cley Cerwyn suggests it is with the defeat of House Bolton, alluding to the Night King as their true enemy, warning that the Night King won't wait out the winter storms, because he is the one who brings the storm.
The Night King is shown leading his army of Wights and White Walkers south, departing the Lands of Always Winter and heading towards the Wall at last.
Sansa Stark and Jon Snow later discuss who the more dangerous enemy is, Cersei Lannister or the Night King, as well as the fact that the Wall still separates humanity from the White Walkers.
Despite her obsession with defeating Cersei Lannister, who sits the Iron Throne, Daenerys Targaryen, who has begun her invasion of Westeros, is told by Jon Snow, who has traveled to Dragonstone, that she will only be ruling a graveyard if the Night King attacks, as he is the true enemy.
Jon takes Daenerys down into the caves on Dragonstone after she agrees to allow he and his people to mine the dragonglass beneath Dragonstone. However, Jon has something much more important in mind: showing her the ancient carvings by the Children of the Forest on the rocks, which depict the Children of the Forest and the First Men fighting the White Walkers together. One of these carvings strongly resembles the Night King leading the White Walkers.
When Bran Stark wargs into a flock of ravens, he flies them beyond the Wall to spy on the army of the dead. He finds the Night King himself leading them, flanked by some of his lieutenants. When the Night King looks up, the ravens disperse, and Bran is pulled out of the warging. He urges Maester Wolkan to send ravens across Westeros informing the high lords that the Night King is on the move to Eastwatch-by-the-Sea.
The Night King is present when the wights battle Jon Snow at the Wight Hunt, who traveled beyond the Wall with a band of warriors to attempt to capture a wight to use as proof for the gathered high lords of Westeros. When Daenerys Targaryen and her dragons come and attempt to rescue Jon and his group, the Night King is given an ice spear by a lieutenant of his and launches it towards Viserion, piercing the young dragon straight through the neck.
Viserion crashes down to the frozen lake, sinking beneath the icy waters. He keeps his gaze on the enraged Jon, who appears on the verge of calling him down to fight directly, but his lieutenant hands him another spear. The Night King throws it at Drogon, but the dragon takes flight and dodges in time.
The Night King later has his wights heave Viserion's corpse out of the water. Once that is accomplished, the Night King advances towards Viserion's corpse and places a hand on his snout, kneeling to the ground. Moments later, the corpse of Viserion is reanimated as its eyes open, now a depthless, icy blue.
The mounted White Walkers lead the gathered army of the dead out of the Haunted Forest to the Wall at last. Riding the reanimated Viserion, the Night King breaches the Wall, burning it away where Eastwatch-by-the-Sea stands, destroying the castle and allowing the White Walkers and their wight army to invade Westeros once more while the Night King flies overhead towards the North.
Before he became a White Walker, the Night King was a First Man that was captured by Children of the Forest, Leaf among them. Leaf pressed a dragonglass dagger into his chest, causing his eyes to turn blue and turning him into the first of the White Walkers. The Children of the Forest created the White Walkers to defend themselves when Westeros was invaded by the First Men, who were cutting their sacred trees and slaughtering them.