Daenerys Targaryen. Jon Snow. Tyrion Lannister. You can probably name a dozen or so major players on Game of Thrones and give a detailed history for each. However, there's one crucial figure in Westeros you might not know much about, but who might end up being one of the most important players of all: Rhaegar Targaryen.

References to Rhaegar have been sprinkled throughout Game of Thrones' five seasons, but Sunday's episode spent a curious amount of time discussing a character who died over a decade ago. And since showrunners David Benioff and D.B. Weiss have been ruthlessly trimming the fat in consolidating George R.R. Martin's books, we don't believe they would devote so much air time to Rhaegar without a reason.

So who is he exactly? Keep reading for our primer on Rhaegar below.

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Rhaegar was the son and heir of King Aerys II, better known as The Mad King, and the older brother of Viserys and Daenerys (Emilia Clarke), who was born after Rhaegar's death. He was popular, intelligent, bookish and a noted musician. Tywin Lannister had planned to marry Cersei to Rhaegar. However, Aerys shot down the idea as being beneath his son. Instead, Rhaegar was married to Elia Martell, Oberyn and Prince Doran's sister. Elia loved her husband and she had two children with Rhaegar, a daughter, Rhaenys, and son, Aegon.

However, as Littlefinger briefly explained to Sansa in Sunday's episode, when Rhaegar won a tourney at the cursed Harrenhal, he passed over his wife Elia and crowned Ned Stark's sister Lyanna as The Queen of Love and Beauty. A year later, Rhaegar allegedly abducted Lyanna, an act which ultimately triggered Robert Baratheon's rebellion. But though the Starks and Baratheons view Rhaegar's actions as a violent kidnapping, according to Targaryen lore, Lyanna loved Rhaegar back and went with him consensually.

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As the rebellion against House Targaryen gained steam, Rhaegar and Robert met in combat during the Battle of the Trident and Robert killed the beloved prince. It was the result of this battle that inspired Tywin, who had until then remained neutral, to launch the Sack of King's Landing. During this siege, Jaime Lannister slit Aerys' throat, becoming the Kingslayer, and the Mountain, under Twyin's orders, brutally murdered Rhaenys and Aegon before raping and killing Elia too.

In other words: Rhaegar was the match that burned down all of Westeros. He was the catalyst to the Targaryen downfall, to the bad blood between Dorne and the Lannisters, to Robert Baratheon's rise to king, to Daenerys' exile and to all the deaths that have occurred as a result. But the most important thing Rhaegar did remains to be confirmed.

If you ask most Game of Thrones fans who Jon Snow's (Kit Harrington) father is, few will say Ned Stark. That's because, as Stannis blatantly pointed out on Sunday, cheating on Catelyn with a tavern whore doesn't exactly fit with Ned's code of honor. So, who are Jon Snow's parents? The most popular theory is that he's the son of Rhaegar and Lyanna. A huge clue in support of the theory, known as R+L=J, is that when Ned discovered Lyanna during the Sack of King's Landing, she entrusted her brother with a secret promise shortly before dying. It's highly likely this promise was for Ned to keep Lyanna's son with Rhaegar safe from Robert's quest to kill every Targaryen. And what better way to do that than taking the infant in as one's own bastard son?

If R+L=J is true, Game of Thrones could be paving the way for Jon to wind up on the Iron Throne, or at least play a very key role in the crowning of whomever does wind up ruling. When Daenerys visited the House of the Dying, she witnessed a vision of Rhaegar telling Elia that he believes their son, Aegon, is "the prince that was promised" and that they must have more children since "the dragon has three heads."

However, prophecies are not an exact science, so it's possible the identity of the prophesied prince could be Daenerys or Jon, (who would be Dany's nephew if R+L=J is true). If the prince was Jon, that would also explain Melissandre's growing interest in him, since the promised prince prophecy seems to be closely tied with Azor Ahai. Either way, it's highly likely Jon is the second of the three dragon heads Dany has been searching for.

So what do you think: Is Jon Snow Rhaegar and Lyanna's son? Is he destined to sit on the Iron Throne? Share your theories in the comments below!