Fan theory: Luke Skywalker, Kylo Ren and Rey are the new incarnations of The Mortis Father, Son and Daughter. And Supreme Leader Snoke knows it.
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Ever since Disney took over the Star Wars franchise, they have declared what is cannon and what is not. Star Wars The Clone Wars cartoon is official cannon. One of its best story arcs was the Mortis one. In this story, after following a 2,000 years old distress signal, Anakin, Ahsoka and Obi-Wan encounter a monolith floating in space. It ended-up being a gateway to the planet Mortis (which exists outside the material world). This planet was home to three humanoid creatures known as “The Ones.” They were The Father, The Son, and The Daughter and were extremely powerful force users. They were also the embodiment of the force. The light side (The Daughter), the dark side (The Son) and the balance between the two of them (The Father).
Mortis is suspected to be the origin point of the force itself. This allowed the father to maintain the balance in the force with ease. But the Father, Daughter and Son were neither Jedi nor Sith. They had powers beyond any other known force user, and they were ancient. The father was looking for the chosen one, Anakin, to take his place. While the Son was trying to use him to escape Mortis and return to the galaxy. At the end of the story arc (spoiler warning) all three members of the family ended up dead.
How is this important to Star Wars Episode VIII: The Last Jedi? Keep reading to find out.