Night King

"The true enemy won't wait out the storm. He brings the storm."

- Jon Snow, regarding the Night King.

The Night King in the TV series, is the leader of the White Walkers, the very first of their kind, and the Bigger Bad of the TV series Game of Thrones. He existed since the age of the First Men. He is also the master of the Wights.

He is a creation of the television series and is not expected to appear in the A Song of Ice and Fire novel series.

In the TV series adaptation he is the apparent mastermind behind their coming invasion. He serves as a minor antagonist in Season 4 and the secondary antagonist of both Season 5 and Season 6.

The religion of R'hllor claims that he and the Walkers are servants of an evil god of ice, archenemy of the Lord of Light, known as the Great Other. Red Priests claim that only Azor Ahai reborn can stop them and the Long Night that Never Ends.

It is shown that the Children of the Forest created him as the very first White Walker to defeat the humans.

Game of Thrones
The episode "Oathkeeper" reveals the fate of the infants, as well as introduce the Night King. A White Walker takes Craster's last son to what appears to an altar where the Night King himself greets the infant, and with a poke to his cheek, turns him into another White Walker.

In the episode Hardhome, in which Jon Snow, now Lord Commander himself, goes to the titular village on a mission with Tormund, to make peace with the remaining wildlings from the army of Mance Rayder fled during the attack of King Stannis Baratheon (all the other wildlings and giants are around beyond the Wall like groups, Craster's wives, and a group at the river Milkwater ) and get them to the Wall. While they are evacuating the village, an army of wight's attack, and a massacre ensues. The Night King and other White Walkers oversee the attack. During the fight, Jon Snow kills as White Walker, which the Night King witnesses. After the fighting, Jon and his men flee with the last of the evacuates, and the Night King walks down the dock. Jon and the Night King make eye contact, and the king has him watch as he resurrects the dead of the battle into new wights.

In the episode The Door, it is revealed that the Night King was the first White Walker and that the White Walkers were created by the Children of the Forest as a weapon against the First Men. During one of his greendreams, Bran sees the undead forces of the White Walkers. He walks through the army until he stands before the Night King who sees him. The Night King grabs Bran by the arm which leaves a lasting mark, even after Bran has woken. This mark allows the Night King to penetrate the security spells around the cave and he sends his undead army inside it. Bran and Meera are able to flee but the Night King and his men eradicate all remaining Children of the Forest as well as the Three-eyed raven who is slain by the Night King himself.

A Song of Ice and Fire
In the novels there's no mention of this character. There is only a legend about a traitor known as the Night's King. George R.R. Martin stated that the Night's King is a legendary figure who's most likely to be dead. There's also no mention of the Others having a leader at all. The Night King from the show has been added only for the TV version of the events. There are different rumors and theories about possible candidates for a new leader of the Others who will lead them during the Long Night that Never Ends.

A possible candidate for the "next Night's King" is Stannis Baratheon, due to his similarities with the Night's King and the fact he took the Nightfort as his seat in the North. Stannis is also said that he will break before he bends. In the House of the Undying Daenerys Targaryen sees in a vision a blue-eyed king with a red sword, who casts no shadow. The description matches with Stannis, who's said to have deep blue eyes, has the magic red sword called Lightbringer, and created two Shadow Assassins with Melisandre. Voices of the Undying Ones call Daenerys "slayer of lies".

In the book version the situation with the wildlings is more complicated and the events at Hardhome is happening "off-screen" as Jon Snow, who's the POV character of this storyline, stays at Castle Black and sends Cotter Pyke and Tormund there.

Trivia

 * The Night King's first appearance on the show was controversial to the fans of A Song of Ice and Fire novels, of which Game of thrones is based off, because there was no implication of his presence in the books.
 * When asked about this, George R.R. Martin claimed: "As for the Night's King (the form I prefer), in the books he is a legendary figure, akin to Lann the Clever and Brandon the Builder, and no more likely to have survived to the present day than they have."
 * He is portrayed by Richard Brake who is also Joe Chill in Batman Begins.
 * He is evidently similar to The Lich King.
 * Night King received the title "The True Enemy", Sauron from Lord of the Rings received almost the same as he referred to be "The Enemy".