This bombshell 'Game of Thrones' theory will probably come true on Sunday's finale

Warning: Major Spoilers ahead for "Game of Thrones," including speculation of future events. 

"A Song of Ice and Fire" fan theories are virtually endless, especially since author George R.R. Martin has yet to publish the final two books, leaving many storylines open-ended and up for interpretation. But one fan theory is so widely believed by the fandom that it may as well be canon. The theory is known as "R + L = J" and centers on the question of Jon Snow's true parentage. 

According to a majority of the fanbase, Jon is not the bastard son of Ned Stark, but the son of Rhaegar Targaryen and Lyanna Stark. 

If true, that could make Jon the rightful heir to the Iron Throne, and the rare descendant of two very powerful houses. Dragon-riding and warging abilities are two of the most magical powers a human can possess in the world of "Game of Thrones," and this would mean Jon has potential for both.

A source close to HBO confirmed with Tech Insider that we'll definitely know who Jon Snow's parents are by the end of season six, which means the finale episode will contain the big reveal.

If you're not familiar with the popular theory, here's what you need to know.

The origin of "R+L=J"

"R + L = J" is one of the oldest fan theories, starting on as far back as 1998, according to some readers. That means fans have been waiting for nearly two decades for confirmation on this idea. The actual theory title is (obviously) a simplified version of Rhaegar + Lyanna = Jon, but by using first initials only people on forums like Westeros.org and /r/asoiaf are able to title their theory discussions without giving away spoilers to those unaware of the idea. 

So, the theory itself. In the timeline of Westeros history, we have to go back to 17 years before the events in season one. Ned Stark and Robert Baratheon were best friends, and Robert was betrothed to Ned's younger sister, Lyanna. 

The king at the time was Aerys Targaryen, known as the Mad King. Aerys had three children: Rhaegar, Viserys, and Daenerys. Prince Rhaegar, the eldest, was in line to inherit the Iron Throne. He was married to Elia Martell, the sister of Oberyn Martell. 

Suddenly, Lyanna and Rhaegar both disappear. When Lyanna's father and older brother accuse the Mad King and Prince Rhaegar of kidnapping her, a war breaks out. Robert and Ned lead the charge in rebellion against the Targaryens. The war ended when Tywin Lannister sacked King's Landing and Jaime Lannister killed King Aerys, earning himself the nickname "Kingslayer."

Robert Baratheon killed Prince Rhaegar at the Battle of the Trident. On orders from Robert and Tywin Lannister, Rhaegar's wife Elia was murdered, along with her two children. Rhaegar's younger siblings, Daenerys and Viserys fled Westeros.

Ned, who married Catelyn before leaving to fight in Robert's war, found Lyanna dead in a place called the Tower of Joy. He returned home with a baby boy named Jon, admitting to his wife that he had an affair while at war. Ned raised the child in Winterfell, giving him the last name Snow to point out his bastard origin.

In the season one finale, Bran and Osha are in the crypts, also looking at Lyanna's grave. He tells the short-version of the story to Osha: "King Robert was supposed to marry her, but Rhaegar Targaryen kidnapped her. Robert started a war to win her back," Bran says. "He killed Rhaegar, but she died anyways." As far as Bran (and the rest of Westeros) knows, Rhaegar kidnapped Lyanna against her will, and Lyanna somehow died as a result. 

But is that what really happened? Let's fast-forward to the events unfolding "Game of Thrones," and see if the "R + L = J" theory has any weight.

It's hard to believe Jon Snow is really Ned Stark's son.

Lord Eddard Stark is probably the most honorable man in Westeros. Nearly everyone knows it, and some - like Littlefinger - take advantage of it. Throughout the series, he is referred to as loyal, dutiful, and highly respected.

So why would a man as noble as Ned break his solemn marriage vow? Some characters on the show wonder the same.

In season two, Jaime taunts Catelyn Stark about Ned's infidelity while being held captive.

"I've never been with any woman but Cersei," Jaime tells Catelyn. "So in my own way, I have more honor than poor old dead-Ned. What was the name of that bastard he fathered? Snow? Now, when good old Ned came home with some whore's baby, did you pretend to love … the walking, talking reminder that the honorable lord Eddard Stark f---ed another woman?"

Though Jaime is clearly trying to push Catelyn's buttons he makes a keen observation. He suggests Jon Snow's conception seems a little out of character for Ned Stark. 

Another concrete time this infidelity is questioned comes in season five. Stannis Baratheon and his wife, Selyse, discuss Jon Snow and his parentage comes up.

"You think highly of this boy?" Selyse asks of her husband.
"Lord Commander of the Night's Watch," Stannis replies.
"And a bastard by some tavern slut," she says.
"Perhaps, but that wasn't Ned Stark's way," he replies.

This is the first time in the show a character outright questions whether Ned is really Jon's father. For those who believe in "R + L = J," Stannis' line was a huge hint toward some confirmation.

As well, Ned was especially secretive about the details of his affair, even from his good friend Robert.

In a season one episode, Robert and Ned reminisce on the good ol' days of their youth, discussing their sexual conquests. "Yours was uh, Meryl? Your bastard's mother?" Robert tries to guess, referring to Jon. Ned's face looks grim. "Wylla," he replies. No other details are given.

"That's it. Must've been a rare wench to make Lord Eddard Stark forget his honor," Robert says. "You never told me what she looked like."

"Nor will I," Ned says. Robert looks taken aback. Ned and Robert were raised together, both ward's under a lord named Jon Arryn. They are practically brothers, and Ned's refusal to confide in Robert is unusual. 

Why would Ned dishonor himself and his wife, especially if the dishonor was the result of a lie?

Ned's extreme sense of honor and duty can only be compromised when it comes to children. 

In the ninth episode of season one, Ned Stark confesses to a treason he knows he didn't commit, and is executed as a result. Varys convinced him to lie and name Joffrey - the bastard son of Cersei and Jaime Lannister - as the true king. Why? Because Sansa's safety depends on it. Ned knows serious harm will come to her if he continues trying to unseat the Lannisters from power. 

Ned values the life of his children above his own honor and glory. It makes sense that he would allow the world to believe that he was once an unfaithful husband if it meant protecting his family. And if Jon truly is the son of Rhaegar Targaryen, he would definitely be in danger - especially from his good buddy Robert Baratheon.

If R+L=J is true, Robert would not have been kind to baby Jon. He loved Lyanna fiercely. How would he react if he found out that she had a child with Rhaegar? Probably not too kindly since that child, half Stark and half Targaryen, would have a claim to the throne Robert sat in. 

Robert made it clear he didn't have much love for the remaining Targaryens in season one. Multiple times he expresses his desire to murder Daenerys. Though Ned explains she is "little more than a child," it all but falls on deaf ears.   

Everyone seems to have Rhaegar Targaryen all wrong.

Early on, book readers and show watchers are led to believe the Targaryen family is ruthless and (sometimes) insane. We know the Mad King burned people alive out of paranoia, and Daenerys' older brother Viserys is portrayed as having unsavory anger issues. With Robert's clear distaste for Rhaegar, and hearing he kidnapped Lyanna, it makes sense people would assume the Targaryen prince was not a good man.

As both the show and book progress, we begin hearing more about the type of person Rhaegar Targaryen was. Ser Barristan Selmy tells Daenerys in season three he was the "finest man I ever met."

Ser Barristan is considered one of the greatest and most honorable knights in Westeros. If he believed Rhaegar was a great man, viewers should consider that high praise. In the same scene, Barristan told Daenerys about the rebellion, and the final battle on the Trident. "When your brother Rhaegar led his army into battle at the Trident, men died for him because they believed in him, because they loved him," he claims. 

Ser Jorah Mormont chimes in, saying Rhaegar fought "valiantly" and "nobly."

In season five, Ser Barristan tells Daenerys even more heartwarming stories about what a wonderful man Rhaegar was. "He liked to walk among the people," Barristan says. "He would sing to them. He was very good ... Rhaegar never liked killing, he liked singing." 

Based on these anecdotes, Rhaegar doesn't sound like a monstrous man who would kidnap a woman for no reason. 

In season four, Oberyn Martell also gives Rhaegar's character more depth when he tells Tyrion a story about visiting King's Landing. Remember, Oberyn was the brother of Rhaegar's wife Elia. According to Oberyn, Rhaegar simply had an affair with Lyanna.

"My sister loved him," Oberyn tells Tyrion. "She bore his children … and beautiful, noble Rhaegar Targaryen left her for another woman."

Oberyn's choice of words makes it sound like Rhaegar and Lyanna had an actual relationship. Could he know more about the events from 17 years ago than the rest of Westeros?

Littlefinger is another character who may know more than he lets on. In season five, Littlefinger and Sansa discuss Rhaegar and Lyanna while visiting the crypts of Winterfell. 

Littlefinger tells her a story.

"Lord Whent had a great tourney at Harrenhall ... Lyanna was already promised to Robert ... the last two riders were Barristan Selmy and Rhaegar Targaryen. When Rhaegar won everyone cheered for their prince ... until he rode right past his wife, Elia Martell, and all the smiles died ... He rode past his wife and lay a crown of winter roses in Lyanna's lap, blue with frost ... How many tens of thousands had to die because Rhaegar chose your aunt?"

"Yes he chose her," Sansa replies. "And then he kidnapped her and raped her."

Instead of confirming OR refuting this allegation, Littlefinger simply smiled knowingly at Sansa, almost as if he was amused by her indignant response.

Worth noting is that this scene takes place in the same episode when Stannis Baratheon questions Jon's parentage, and when Barristan tells Daenerys about Rhaegar's love for singing. The showrunners really seem to want the average viewer to question Sansa's assumptions about Rhaegar.

Other clues about Jon's parentage in the show:

Context clues about individual characters aside, the "Game of Thrones" showrunners have also slipped in a few symbolic references to Jon's true parentage. 

For one, there's a vision Daenerys had at the end of season two. She walks through the throne room in King's Landing, which is filled with snow. You know, like Jon Snow? It could just mean winter is coming, but we love the potential double-meaning.

But the kicker is when she walks right out of the throne room and out from underneath the Wall - where Jon Snow lives at Castle Black. Daenerys has never seen the great ice wall, and doesn't know who Jon Snow is but her vision seems to be drawing her in his direction.

Another fun symbolic moment happened in season five when Maester Aemon is talking to Sam about Daenerys. Aemon is a Targaryen himself, Daenerys' uncle. "A Targaryen alone in the world … it's a terrible thing," he says.

And who walks in at that very moment? Jon Snow. Not only may he be half-Targaryen, but he is practically alone in the world with no family left that he knows of.

Ned also uses some tricky wordplay when discussing Jon's mother with him early in the series. "You may not have my name, but you have my blood," he says. Notice how he intentionally avoided the word son. But he wasn't lying: the blood of Lyanna Stark may run through Jon's veins. 

So is "R + L = J" true?

We know Prince Rhaegar and Lyanna disappeared together, starting a war as a result. Though many believe he kidnapped Lyanna, others seem to think it was an actual love affair. Based on character clues from the men involved, it would have been highly unusual for Rhaegar to spark a war over a violent kidnapping, and even more unlikely that Ned broke his marriage vows and fathered a bastard. 

Instead, it is likely that Rhaegar and Lyanna were in love and had a child together. And by turning to the books for extra proof, Jon's parentage becomes pretty clear.

Ned was the one who found Lyanna after the battle was over. In the first book, "A Game of Thrones," Ned has a dream-like flashback to a place called the Tower of Joy. Inside the tower, he finds Lyanna in a "bed of blood," and she repeatedly tells him "Promise me, Ned, promise me." Fans believe the bloody bed means she died as result of childbirth. 

And the promise Ned kept? It's believed that was to protect his baby nephew, the child of Lyanna and Rhaegar, from the wrath of Robert Baratheon and to raise him as his own son, no matter the cost.

Fans have also kept a sharp eye out for clues from author George R.R. Martin himself. In one interview, he spoke about fans picking up on clues he had dropped in the earlier book, possibly referring to Jon's parents. 

Martin also made the show's producers, D.B. Weiss and David Benioff, tell him who they thought Jon's parents were before moving forward with optioning "A Song of Ice and Fire" to HBO. "He asked us, 'Who is Jon Snow's mother?'" Weiss said. "We had discussed it before, and we gave a shocking answer.  At that point, George didn't actually say whether or not we were right or wrong, but his smile was his tell."

What this means for Jon

Well, first off, it makes him the literal "Song of Ice and Fire," because of his ties to the northern Starks and the dragon-riding Targaryens. This also means he is technically Daenerys' nephew. In terms of claims to the Iron Throne, it would depend upon whether or not Rhaegar actually married Lyanna, making Jon a legitimate royal child. 

Rhaegar was already married to Elia Martell, but Targaryens have a history of taking multiple wives, so it is possible. If that's what happened, Jon's claim to the Iron Throne is technically stronger than Daenerys' claim. He's the direct male descendant of the Mad King, whereas she is the sister of the former prince.

Fans may be in a for a huge surprise when Jon's true parents are revealed in the season six finale via a flashback to Ned at his sister's deathbed. We've already seen the beginnings of the Tower of Joy scene through Bran's flashbacks earlier in the season. The finale episode will likely show Ned finally entering the tower, and seeing Lyanna with baby Jon Snow.

The "Game of Thrones" season six finale airs on Sunday June 26 at 9 p.m. EST. All hopes are hanging on Bran to reveal the truth behind "R + L = J" - a theory confirmation that is decades in the making.