By now most fans know that HBO's hit show 'Game of Thrones' is based on an original book series by George R.R. Martin. What they might not realize is that fans of the books have amassed dozens and dozens of predictions and theories that cover a wide range of characters.Martin began writing the saga titled "A Song of Ice and Fire"nearly two decades ago, this means that diehard fans of the novels have had more than enough time to come up with a range of theories about current characters and plots and where they all might be headed.Some of these guesses seem close to the mark, like the widely accepted assumption about Jon Snow's true parentage. Martin has also implied a fan has correctly guessed his planned ending for the entire series.Other theories have more of a conspiracy feel to them ' loosely connecting threads and unabashedly stretching the imagination. For example, the suggestion that Daario, the handsome sellsword Daenerys is currently having an affair with, is actually Benjen Stark ' Ned Stark's brother who went missing from the Night's Watch.But there is one theory that rises above all others in the realm of insanity (bordering on genius). We're talking about the 'Cleganebowl."It goes a little something like this: Sandor 'The Hound' Clegane is going to return to King's Landing in order to fight his brother, Gregor 'The Mountain' Clegane, to the death in a trial by combat for Cersei Lannister.This may sound a bit bizarre to those who have been following the show. Wasn't the Hound left for dead in season 4' And isn't The Mountain lying under a sheet in some creepy guy's laboratory' Even if they're both alive, why should I care if they fight' All this, and more, to come. First let's look at why this theory is relevant right now based on events taking place in season five.A champion for the crownIn episode seven, we saw the High Sparrow (the leader of the Faith) arrest Cersei and throw her into a cell to await trial. We learned afterward she wasimprisoned for her role in King Robert's death as well as her incestual sexual habits with her brother Jaime.Now, on the show so far, we have never seen a trial concluded in a purely judicial manner. Tyrion has twice stood accused of crimes, and both times he demanded a trial by combat. A trial by combat allows the defendant to select a 'champion' to represent him in a fight to the death against the selected champion of the prosecution. The guilt of the defendant is determined by whether their champion wins or loses.*Minor spoilers ahead now for show-only fans*From the books, we know that Cersei will demand a trial by combat. And her champion' Well this is where it gets a bit complicated.The Undead MountainBack in season four, we watched in horror as Oberyn Martell battled against Gregor Clegane (commonly known as The Mountain). Oberyn managed to stab The Mountain with a poisoned spear, but it wasn't enough to win the fight. The Mountain crushed Oberyn's head in with his bare hands, and won the trial by combat.Since then, we have only had glimpses of The Mountain in what appeared to be various stages of experiments. Here Qyburn examines the poisoned wound, and reassured Cersei that he had a plan to keep The Mountain alive.When he was explaining to Cersei the gist of his plan, he warned her: "You should know. The process may change him, somewhat.'Next, in season five episode three, we got a glimpse into Qyburn's laboratory when Cersei asked him to send a letter. While there, Cersei asked 'How's your work coming along'' (referring to the Mountain's "healing").Qyburn replied 'Better than expected, but still a ways to go.'Then we see a large shrouded figure in the background, and suddenly it writhes under the sheet.So we know that The Mountain is still alive in some capacity. In the written series, Cersei writes to Qyburn from her prison cell and tells him to appoint a new member of the Kingsguard ' one that can serve as her champion. Qyburn presents a large knight named Ser Robert Strong.It is extremely likely that Robert Strong is none other than the zombified version of The Mountain that Qyburn has been experimenting on. So, Cersei has her champion.The Faith's ChampionIn order to understand why fans think Sandor 'The Hound' Clegane will be named the Faith's champion, we must explore a tangential theory known as 'The Gravedigger.' YouTube user Alt Shift X made a popular visual explanation of the theory ' you can watch the full video here.The basics of this theory center around The Hound, and how in both the books and the show he is never actually seen dead. The last time we saw him on the show was in the season four finale, when Arya left him grievously wounded. This same scenario happens in the books, though the fight that leaves him injured is different.In the books, Brienne of Tarth is wandering the country, looking for The Hound and (she hopes) Sansa or Arya. She goes to a place called the Quiet Isle, a sort of monastery for brothers of the Faith, and speaks to the Elder Brother.This Elder Brother knows much about The Hound, and admits that he came across him while wounded and did his best to help him. Brienne then notices a very large man at the monastery who is digging graves. His head and face are nearly completely covered, but Brienne can tell he is limping from what could be a healing leg wound (the exact type that The Hound sustained). The Hound's horse is at the Quiet Isle stables ' another hint that The Hound is likely nearby. His horse is notorious for being uncontrollable by any other than The Hound himself.Lastly, the Elder Brother is very vague when discussing The Hound's supposed death. As Alt Shift X puts it, 'The most he'll say about Sandor Clegane is that he's 'at rest.' So what the Elder Brother could be getting at is that the hateful, violent part of Sandor Clegane, called the Hound, is metaphorically dead, leaving Sandor Clegane 'at rest,' and most importantly for us, literally alive.'So the Gravedigger theory infers that not only is the Hound alive, but he has now been brought into the Faith and serves them as Sandor Clegane.Cleganebowl and the hype that won't dieThese two separate incidences ' zombie Mountain and grave digging Hound ' have combined to give birth to the Cleganebowl theory.It started in 2013, when a 4chan thread about 'Game of Thrones' prompted a user to submit their theory.The title for the theory likely was inspired by the 2013 NFL Superbowl when coaches of the opposing teams also happened to be brothers: Jim and John Harbaugh. Football fans and sports announcers quickly nicknamed the game 'The Harbowl.'Since the Hound and the Mountain are brothers, and the theory assumes they will have an epic fight, Cleganebowl is the best fitting title. They share the same last name ' Sandor and Gregor Clegane ' and this trial by combat would basically be the equivalent of a Westerosi Superbowl.This original 4Chan thread references the "Valonquar" prophecywhich we explained morehere. The gist of it is that Cersei went to a fortune teller when she was younger, and was told that she would be killed by "the valonqar." That is a term that means "younger brother," so the Cleganebowl theory postulates that the Hound is the younger brother in the scenario, and his victory will lead to Cersei's execution.Believers in the Cleganebowl are a unique brand of fans. The tagline that generally follows any online allusions to the involved characters is a simple: 'GET HYPE.' An entire subreddit is dedicated to the converted believers, and this Google search history shows the spikes of interest over the years.Cleaganebowl-subscribers have a flair for the unique. There is a group of YouTube videos that mash up footage from the show with glaring strobe-text and an intense hip-hop/dubstep/electronic music soundtrack. Here are some of the top viewed ones [Warning: NSFW language and loud music]Cleganebowl is such a popular theory not just because of its outlandish fanbase, but also because it would actually be a really awesome scene to watch play out. The Hound and the Mountain are two of Westeros' renowned fighters with a lot of anger to spare. They have an intense rivalry (remember it was the Mountain who disfigured the Hound's face when they were children) that would make a fight to the death extremely ins tense.Airhorns and Cleganebowl-hype-meters aside, there is sufficient evidence for the two main components of the theory. The Mountain could definitely be Ser Robert Strong, and will represent Cersei in her trial-by-combat. It is also likely that the Hound is alive and living on the Quiet Isle.So far, season five has alluded heavily to the Mountain's return.Whether the Hound resurfaces is another story, but those who believe it will happen will certainly wait with bated breath to see if this insane theory comes true.SEE ALSO:The 'Game of Thrones' season 5 premiere hinted at a huge fan theoryAND:'Game of Thrones' creators say they already know the ending to the showJoin the conversation about this storyNOW WATCH: This guy just made a sword out of a nail and it's worthy of 'Game of Thrones Click here to read full news..