Cleganebowl is a fan theory surrounding a hypothetical event in the future plot of George RR Martin’s epic fantasy novel series A Song of Ice and Fire which posits that Sandor Clegane ("The Hound") and his brother, Gregor Clegane ("The Mountain"), would be pitted against each other in Cersei Lannister's trial-by-combat, with the former representing the Faith, one of the two major religions in the Seven Kingdoms, and the latter fighting on behalf of the accused queen.
George R.R. Martin published A Game of Thrones, the first book in the A Song of Ice and Fire series, which the HBO show Game of Thrones is based on, on August 6th, 1996. The first documented mention of the Cleganebowl theory online comes from an archived 4chan thread started by an anonymous user on March 20th, 2013.
Note: this section contains spoilers
In George R.R. Martin's A Feast for Crows, the fourth novel in the A Song of Ice and Fire series that is also expected to be featured in the fifth season of Game of Thrones, Cersei Lannister gets arrested and imprisoned for plotting a series of murders. Upon facing the court, Cersei elects for trial by combat. The theory speculates that Sandor Clegane will survive his fall and return as the The Faith’s champion in the duel against Gregor, who had previously fought on behalf of Cersei during Tyrion Lannister's trial by combat.
The theory has been included in many Game of Thrones fan theory roundups. The theory was included in a fan roundup published on April 20th, 2014, by The Daily Dot titled "The ultimate guide to 'Game of Thrones' conspiracy theories."
On June 11th, 2014, the subreddit r/cleganebowl/ was created. In less than a month the subreddit gained over 300 readers. On June 18th, 2014, YouTuber Shane Ysais uploaded a video titled "CLEGANEBOWL," which alleges that the theory is fact. In less than a month the video had gained over 2,000 views.
Sandor Clegane's Return
On June 5th, 2016, Game of Thrones' Episode 7, Season 6 "The Broken Man" aired on HBO, in which Sandor Clegane reemerges for the first time since being severely wounded in a battle with Brienne of Tarth and left behind by Arya Stark in the fourth season's finale episode "The Children." In the episode, it is revealed that Sandor survived the seemingly fatal wounds after getting rescued by Brother Ray, a reformed killer and leader of a village in the Riverlands, and has been living a quiet life there. However, as Clegane begins to form a bond with Brother Ray, he returns from scavenging for water in the forest only to discover all of the villagers, including their leader, ruthlessly murdered by bandits of Brotherhood Without Banners. Enraged, Clegane picks up an axe and leaves the village behind, thus departing from the path of nonviolence that Brother Ray had once preached. Following the airing of the episode, many fans of the series welcomed the return of Clegane as the likely beginning of a build-up to the highly anticipated duel.
On May 12th, 2019, episode five "The Bells" of season eight of Game of Thrones premiered. In the episode, the long-anticipated duel between brothers Sandor and Gregor Clegane takes place in the location of Red Keep. The sword and fist combat results in both brothers plummeting off the tower into the flames engulfing the lower grounds, presumably leading to the deaths of both.
 The Daily Dot – The ultimate guide to 'Game of Thrones' conspiracy theories