Did you watch Game of Thrones on HBO Go last night? Then you may have gotten a surprise bit of information that is so monumentally huge, it's not just a spoiler for the show but it's possibly a spoiler for the A Song of Ice and Fire books as well. CONTINUE AT YOUR OWN RISK.
Seriously, last chance.
Okay. Remember that crazy spiky-horned head of the White Walkers that showed up at the end of "Oathkeeper" last night and blew our minds? Well, although the scene was something we haven't read in the books, we've read about the character before.
According to legend, the Night's King lived during the Age of Heroes, not long after the Wall was complete. He was a fearless warrior, who was named the thirteenth Lord Commander of the Night's Watch. Later he fell in love with a woman "with skin as white as the moon and eyes like blue stars", he chased her and loved her though "her skin was cold as ice", and when he gave his seed to her he gave his soul as well. (Her description matches that of the Others.)
He brought her back to the Nightfort and after the unholy union, he declared himself king and her his queen, and ruled the Nightfort as his own castle for thirteen years. During the dark years of his reign, horrific atrocities were committed, of which tales are still told in the North. It was not until his own brother, the King in the North, and Joramun, the King-Beyond-the-Wall, joined forces that the Night's King was brought down and the Night's Watch freed. After his fall, when it was discovered that he had been sacrificing to the Others (possibly in similar way to Craster), all records of him were destroyed and his very name was forbidden. It is likely this led the lords of the North to forbid the Night's Watch to construct walls at their keeps, ensuring the keeps would always be accessible from the south.
In the books, Old Nan told Bran the story of the Night's King, and despite rumors to the contrary says he was a Stark, a brother to the King of Winterfell, and that his name was Bran as well.
If that was the Night's King last night, then the legend is true, and what we thought about the Others/White Walkers has changed dramatically. And if this legend is true, it potentially means a lot of the legends of the world of Westeros are true, which could have major implications down the line (I'd say that magical Wall-destroying Horn of Joramun is definitely real). The potential thematic elements alone — there's no way our Bran Stark doesn't have some serious business to do with his undead namesake — are mind-boggling.
Now, HBO Go calling the character the Night's King could have been an accident in that someone needed a name for the White Walker head honcho, and Night's King fit the bill, but it seems far more likely that the accident was that they revealed the true name of this major character. Either way, HBO quickly removed the name from Go.