The Night King was once a mortal man, one of the First Men who waged a lengthy war with the Children of the Forest. As the Children were losing the war, a small group of greenseers captured the unfortunate man and bound him to a heart tree to be used for a terrible purpose. Leaf, who was among the group, pressed a cursed dragonglass dagger into the captive’s chest, causing his eyes to turn blue and turning him into the first of the White Walkers. Thousands of years later, Leaf explains to 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 down and slaughtering the Children of the Forest.

However, the White Walkers soon turned on their creators and began what was known as the Long Night. Though the Long Night ended upon the First Men and Children’s victory in the War for the Dawn, the Night King survived and retreated with the rest of his kind to the Lands of Always Winter, where they hid and faded into legend.

The Night King is a legendary figure, known by name among, at least, some noble houses in the North.

His Abilities:

Cryokinesis: Along with his race, the Night King was a being of pure ice and cold, and thus could freeze anywhere he went and anything he touched.

White Walker conversion: The Night King could turn male human babies into White Walkers by pressing the tip of his finger to the baby’s cheek. The child’s skin would begin to pale and its eyes would turn the same blue as the other White Walkers.

Necromancy: The Night King could raise and reanimate unburnt corpses as wights. He did not require physical contact to do so, and could raise thousands of wights at a single time just by lifting his arms.

Enhanced strength & Enhanced durability

Weapon shattering: The Night King’s touch could presumably shatter regular metal weapons, as with most White Walkers

Marking: The Night King could mark someone so that he would know exactly where that person was, and no magical boundary could stop him from pursuing a person so marked. He proved this when he marked Bran immediately prior to the battle at the cave of the Three-Eyed Raven.

Minor seismic control: The Night King could cause fissures in the ground.

Detecting Wargs: The Night King seemed to be able to sense when animals are controlled by a warg, just like he was able to spot Bran in his vision. When Bran was scouting beyond the wall using a flock of ravens, the Night King stared at the ravens and Bran was pulled out of his warging immediately. It was not immediately clear whether this was because the Night King could sense Bran due to his mark, or if he could do this to any warg

Fire immunity: Just as with the White Walkers under his control, the Night King was immune to fire due to the extreme cold he radiated. During the Wight Hunt, he was seen walking through a wall of dragon fire unharmed, his mere presence causing it to flicker and go out. He was also completely unharmed by direct dragon fire, as he showed no signs of damage or weakness after being completely engulfed in Drogon‘s flames during the Battle of Winterfell..

Dragon riding: Despite having no connection to the Valyrian bloodline and little to no practice, the Night King appeared to be fully capable of riding a dragon with little risk of falling off, though this could have been attributed to him having mental control over his steed, the undead dragon Viserion, who he personally killed and then resurrected as a wight.

Immortality: The Night King was created by the Children of the Forest during the Wars of the First Men and the Children of the Forest and remained alive for about 10,000 years. It can be assumed the magic that transformed him also gave him immortality.

Weather Manipulation: The Night King could create massive blizzards, as shown in the Battle of Winterfell. He used this blizzard to instantly extinguish Drogon and Rhaegal‘s dragonfire.