Humans and devils can not exist in the same world, there would be riots simply because they couldn't get along.
This is why devils live below us in the pits of the underworld, and the humans live on top. The two worlds are quite different but also quite similar in the way that there are two of the same person in the universe, one in the human world and one in the underworld.
One devil, Ariel - pronounced ah-riel, has always wanted to see the above world, he actually happens to be the prince of the underworld, Satan being the king of course. He manages to slip his way out of there one day - having one task to do - kill whoever it is that looks like him and take on their role in the human world.
David Dobrik is a dick. He's a dick who managed to latch onto other peoples success and happens to now be more famous than them.
His friends used to love him, but realise that the fame must have gone to his head. He's different now, not the same David they used to love.
When he somehow takes a wrong turn in his Tesla and feels drawn to the desert, he meets someone who bears quite the resemblance to the guy. That was his last thought before everything went black.
With the real David out of the picture, Ariel - the new David, takes his place.
Looks like it's clear skies from here on out.