Ned Stark may have kept his promise to Lyanna, but he couldn't keep his promise to Jon Snow. This heartbreaking promise to his supposed bastard son was made back in Game of Thrones Season 1, Episode 2, 'The King's Road.'
Now that HBO have dismissed the ideas that Aerys (or even NED!) could be Jon's real father and confirmed that Rhaegar Targaryen + Lyanna Stark = Jon, this conversation is all the more poignant. Any true Game of Thrones fan will feel the feels at the way Ned's voice cracks when he says:
"The next time we see each other, we'll talk about your mother. I promise."
Now we know he's not just talking about some random woman in a tavern sometime in his youth. Ned's talking about his beloved sister Lyanna, who he saw die in his arms (in the Season 6 finale).
During Season 1, I always wondered why Ned let Jon take the black so easily. It's a massive thing to swear your life away to The Wall, and it felt like Ned had been swayed by Catelyn's resentment of his 'bastard son' too quickly. However, knowing that Jon Snow is a Targaryen, The Wall was the safest place for Jon. Robert Baratheon, who loved Lyanna Stark, hated the dragon born so much he would have had little Jon Snow killed.
''I will kill every Targaryen I can get my hands on, until they are as dead as their dragons, and then I will piss on their graves'' - Robert Baratheon
Even Targaryen-hating Robert never had Maester Aemon killed, however. True, he was an old man, but his lineage was known throughout the Seven Kingdoms. Yoren also attempts to take one of Robert's bastards, Gendry, to The Wall to escape Joffrey and Cersei's culling of Robert's illegitimate offspring in King's Landing.
Now Bran knows about Jon's true parentage, what will happen to Westeros's biggest bastard, Jon Snow? Is Bran the only one who knows or does one of Ned Stark's oldest friends know?