Though all the footage featured in the teaser is old — it's a greatest hits of Game of Thrones deaths and dramatic moments, with a heavy emphasis on Jon Snow — the dialogue is new and must be from the upcoming season.
It appears to be an exchange between Bran Stark (Isaac Hempstead Wright), who hasn't been seen since the end of season four, and his new mentor, the Three-Eyed Raven. The Raven will now be played by the legendary Swedish actor Max von Sydow (whom you may remember from such classic films as The Seventh Seal and Hannah and Her Sisters), and we hear von Sydow's sonorous voice as this teaser begins.
When last we saw Bran, he had followed his prophetic visions to the Three-Eyed Raven's cave in the far north, beyond the Wall. Now, as readers of George R.R. Martin's most recent book A Dance With Dragons know, Bran is learning how to use his psychic powers.
"We watch. We listen. And we remember," the Raven counsels Bran in the teaser. "The past is already written. The ink is dry." As he says this, we see season five's shocking cliffhanger, with Jon Snow bleeding to death.
But then we flash to footage of Bran, Jon's brother, with whited-out eyes signaling the use of his powers. "They have no idea what's going to happen," Bran says in the voiceover.
This is, of course, a nod to the debate over whether or not Jon Snow will remain dead. The series has already teased Jon's possible return by spotlighting him on its poster for the new season. And back in June, I wrote about why so many fans believe Jon will be back in season six. To see whether and how it will happen, though, we'll have to wait until Game of Thrones returns in April. (As for Martin's next book, The Winds of Winter, there's still no word on whether he'll manage to complete it before the start of season six, which is expected to spoil large chunks of it.)