Did HBO accidentally reveal a major ‘Game of Thrones’ spoiler?

Warning: Potentially major “Game of Thrones” TV and book spoilers ahead. Stop reading now if you don't want to know!

Just who was that crowned White Walker at the end of Sunday night's episode of “Game of Thrones”?

Well, for a brief period after the episode aired, HBO's official episode synopsis identified the character as the Night's King. He's a character long speculated about by fans of George R. R. Martin’s “A Song of Ice and Fire” novels.

"A White Walker claims the baby and rides to a city of ice," the original synopsis read. "The child is presented at an altar, where the Night's King greets the infant and lays a finger on its cheek. The baby's eyes turn White Walker blue.”

Realizing that they had slipped up, HBO quickly altered the synopsis, changing mention of the “Night’s King” to simply “a Walker,” but not before an eagle-eyed Reddit user could take a screen shot.

Who exactly is the Night’s King?
A former Lord Commander of the Night’s Watch, the Night’s King is said to have been a Stark of Winterfell also named Bran. The legend says that he betrayed his vows after falling in love with a woman "with skin as white as the moon and eyes like blue stars" and declared himself king of the Nightfort. Giving up his soul in the process, the Night's King committed many atrocities during his reign before being deposed by an army of both Northerners and Wildlings. If you're curious to know more, you can read an in-depth description of the character over at A Wiki of Ice and Fire, but we suspect it's not the last viewers will see of the Night's King.

Why is this a big deal?
George R.R. Martin has only released five books in the "Song of Ice and Fire" series, and by showing the Night's King on-screen, the HBO series is now treading in territory not yet covered by the novels. In the books, a legendary character called the Night’s King is mentioned in bedtime stories told to the Stark children, but having him actually appear on screen in the TV show is a complete game-changer. The episode’s spine-chilling ending confirms that not only is the Night’s King real, but apparently he’s leading the White Walkers and building an army out of sacrificed children.

Surprised by the ending of "Oathkeeper? Got your own theory about the Night's King? Let us know in the comments.