He may no longer be a threat, thanks to Arya Stark, but the Night King continues to be one of the greatest mysteries in Game of Thrones. Although his origins have been pretty much revealed in the sixth season, there is still much confusion about who he truly is and if he is also the legendary Night's King. However, it appears that the Night King is completely different from the Night's King that is told in wildling legend.
In the TV series, the Night King was first introduced in the fourth season where Bran Stark sees him in a vision. The Night King is shown picking up Craster's baby from an altar and turning it into a White Walker. His powers are once again shown in Season 5 when he resurrects every corpse at Hardhome in front of a horrified Jon Snow. However, it isn't until the sixth season that we learn the Night King was created by the Children of the Forest as they took one of the First Men, tied him to a Weirwood tree, and pushed a dragonglass dagger into his chest.
So how is he different from the Night's King? According to wildling legend, the Night's King was actually the thirteenth Lord Commander of the Night's Watch. He allegedly fell in love with a cold woman with blue eyes who is believed to be a White Walker and together they ruled over the Wall when he brought her to the Nightfort. However, he was finally defeated when the Free Folk and House Stark worked together.
It is unclear if the Night's King truly did exist or if he was a mere legend. Nevertheless, George R.R. Martin himself may have confirmed that he is completely different from the Night King.
The author of A Song of Ice and Fire was previously asked why the Night King is referred to as such without the possessive S on the show. Martin's response was vague but somehow distinguished the difference.
"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 wrote.
Considering that the Night King was around until Game of Thrones Season 8 Episode 3, it's safe to assume that the Night's King had died long before the Battle of Winterfell.
Related: Game Of Thrones Fan Art Depicts Arya Stark As Mortal Kombat 11's Scorpion