Who among us didn't breathe a huge sigh of relief when Benjen Stark (Joseph Mawle) swung his fiery mace to save Jon Snow (Kit Harington)? As angry zombies descended on Jon, who was cruelly outnumbered, Uncle Benjen came out like Gogo Yubari to save the day. Honestly, it could be criticised as lazy screenwriting, but I'm just happy to see my favourite half-Targaryen sadboi live to brood another day.
But how and why did Benjen Stark show up at precisely the right time? What was the most undead Stark doing at the arrowhead mountain?
According to this Reddit theory (and I think it is very plausible), Benjen knew to help his nephew because Bran sent him. If you'll recall, in Season 6, he appears in the woods to help Bran (Isaac Hempstead Wright) and Meera (Ellie Kendrick) escape a group of wights, who had just killed Hodor and Summer. Uncle Benjen said that the Three-Eyed Raven "sent for" him. Meera counters this, and says that the Three-Eyed Raven is dead, but Benjen says gravely that Bran is the Three-Eyed Raven now. Presumably, it was Bran who called for Benjen, even if he didn't realise he was doing it. Bran's powers are so omniscient that he can seemingly call for help in his vision.
Back at Winterfell, Bran has the ability to see all of time: the future, the present, the past, even parts of time that never happened. If this theory follows, Bran was watching Jon Snow and the Westeros Avengers undertake their massively dangerous trip beyond the Wall — and sent Uncle Benjen to help when Jon Snow needed it the most.
Of course, as a commenter notes, Benjen Stark could have just noticed giant dragons flying over overhead, but this theory seems more interesting, given that we know that Benjen is receptive to distress calls from the Three-Eyed Raven. Of course, now we'll never know. Benjen was finally killed when the wights descended on him, and didn't explain how he ended up there in the first place. Another Stark dies, but because of his sacrifice, another Stark lives on.