When the leader of the White Walkers (first seen in "Oathkeeper") was dubbed the Night King by the producers, confusion set in amongst some fans as to the relationship between two characters. After "Hardhome" aired, Game of Thrones Wiki contacted George R.R. Martin asking whether to treat the White Walker referred to as the "Night King" relative to the ancient Lord Commander known as the "Night's King" - if they are the same character, or if "Night's King" is a title that can be held by different characters, like "King in the North." He was also asked if it was significant that Benioff and Weiss refer to him as the "Night King", without a possessive "S". Martin cryptically avoided the first question, but said he prefers the spelling "Night's King":
"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."[2]
"The Door" revealed that the Night King was "created" by the Children of the Forest during their war with the First Men (i.e., before the Night's Watch was founded). This would suggest they are indeed separate characters.