Yes, the HBO show calls him the Three-Eyed Raven, but whichever corvid you prefer, this guy is a major mystery in the Game of Thrones universe. To begin with, he seems to be a literal crow with three eyes. After lil' Bran Stark gets pushed out of a window, he starts having weird dreams about a black, three-eyed bird, flying around and easing him out of his coma. The crow seems to know what's going on all over Westeros, like showing Bran his dad, Dead Ned, just after he's been executed. But it's not until Professional Creepy Child Jojen Reed shows up in Winterfell that Bran realizes his dreams aren't just any ordinary corvid dreams. Jojen has visions of the Three-Eyed Crow, too. And he believes that the real Crow is up north, beyond the Wall.
So Jojen and his sister Meera head north with Bran and Hodor, fleeing Theon's nonsense at Winterfell. They make it beyond the Wall with the help of Sam, Gilly, and Coldhands the Friendly Zombie. After some fun wight-fighting action (and Jojen's death on the TV show), the gang manages to find a bunch of knock-off hobbits hiding out in a cave. And there's the Three-Eyed Crow in the flesh... except he's not a giant, magical bird. He's an old albino dude who has already partially turned into a tree. And he says he used to be a lord named Brynden.
If you're a nerd like me, here's where you gasp in shock.
But if not... you're probably wondering who the heck Brynden is, and why you should care.
You see, way before Ned and Cersei and Dany and Jon, there was another civil war in Westeros. The Targaryens got all bent out of shape because King Aegon Targaryen IV decided to legitimize all of his many bastards on his death bed. Brynden Rivers was one of these bastards, known as Bloodraven for his red birthmark and his all-around creepy vibe.
Brynden stayed loyal to his "true born" brother, Daeron Targaryen, who took the throne when the king died. But their oldest bastard brother, Daemon Blackfyre, was not cool with that, and claimed the throne for himself. There was a big rebellion, where Brynden lost one of his eyes fighting yet another bastard brother, Aegor "Bittersteel" Rivers. Both Brynden and Bittersteel had the hots for their half-sister Shiera Seastar, because, y'know... it's Game of Thrones. Finally, Brynden killed the "false" king Daemon on the battlefield, and the rest of Team Blackfyre fled across the Narrow Sea to kick it in Essos, found the Golden Company, and launch an attack on Westeros every couple of years.
Lord Brynden "Bloodraven" Rivers was then Hand of the King for a while, and crushed Blackfyre Rebellions whenever they popped up. But when a Targaryen king bought the farm with no clear successor, everybody decided to get together and talk it out (for once) in a Great Council. The current Blackfyre heir over in Essos wanted to come too, to peacefully and politely join the conversation and bring the whole Targaryen family back together.
Brynden was all like, "Yeah that's totally cool, we definitely won't kill you immediately," but then as soon as the Blackfyre kid rolled up in King's Landing, he was seized and beheaded by Brynden's men. Brynden got in major trouble for that move, and wound up shipped off to the Wall for killing this dude so impolitely.
At the Wall, Brynden rose through the ranks to become Lord Commander, before mysteriously vanishing while out on a ranging mission.
No one ever heard from him again... until Bran and company found him way up north, super old and living the magical tree life.
It seems that Brynden was always a "greenseer" to some extent, meaning he could see the future. People thought he was a sorcerer back in the day, although that was probably just because he was a goth kid with red eyes. He may have always had a connection to the Old Gods, too, since his weapon was a weirwood bow. But we don't know exactly what happened to him up north that changed him from an angry, creepy Targaryen bastard into a wise, creepy, future-seeing tree man. He's lived far longer than anyone should, and the weirwood roots have grown into his body. The Children of the Forest are cool with him. He's also able to see through the eyes of animals, and to see through the trees themselves, looking back through time as well as dreaming of the future. He claims that you can't change the past, only look at it... but if the HBO show is any indication, Bran's about to take that rule for one heck of a test run (sorry, Hodor).
It also seems like Bran is next in line to take Brynden's place as this strange tree man... so let's hope that that's a good thing. Brynden has started to train Bran in the ways of the Three-Eyed Crow. On the GoT TV show, it seems like part of those duties involve fighting the White Walkers.
But in the books, Melisandre has also had visions of Brynden in the fire, as the enemy to all things light and fiery:
A wooden face, corpse white. Was this the enemy? A thousand red eyes floated in the rising flames. He sees me. Beside him, a boy with a wolf's face threw back his head and howled.
Does that make Brynden evil? Or is he just the morally grey ice to Melisandre's morally grey fire? Is he really working against the Others, or is his job description a little more nuanced than that? And if the Targaryen loyalist Brynden Rivers is still in there, along with all the creepy tree nonsense... how does he feel about putting a true-born Targaryen back on the throne?