The whole story - A theory
Alright, everyone. Here’s my take on what I think happened, chronologically…
Mark and Selene fall in love and are married. Colonel and Selene have an affair, and Mark gets suspicious and hires Detective, who finds out what’s going on. Selene eventually leaves Mark for Colonel, but Mark wants to “remain friends” in order to exact revenge. Mark becomes a celebrity.
Proof of affair (it’s not much, but I feel it adds to the story):
Mark begins to dabble in the occult as part of his revenge plan. He buys this new mansion. By this time, he is deep into some dark shit, which is why Selene has always had a bad feeling about the house. Darkiplier is forming.
Mark’s plan for revenge is complete. He invites everyone to the party. He invites the Colonel because, revenge. He invites Damien because they’re close friends and he’d be easier to possess, he invites the Detective so that the Colonel will be blamed for the murder, and he invites us because we are spiritually attuned. (Not super confident on the “us” reason.)
So, everyone arrives and is partying it up. However, the butler is serving spiked drinks, either at Mark’s command or, more likely, without knowing they’ve been spiked.. I mean, we take one sip, trip BALLS, forget 99% of the night, and pass out. Seemingly so does everyone else. Additionally, whatever is in these drinks causes violent tendencies and anger - we flip people off, Colonel brings out his gun, the Detective straight-up punches us. I’m guessing some of the drinks have Ketamine, PCP, or something similar. During this time where everyone is out of it, Mark lures the Colonel elsewhere, and uses the drug’s effects to provoke the Colonel into killing him. The Colonel remembers nothing, or hopes it was all a bad dream. Mark possesses Damien.
Note: it is possible that Mark simply killed himself in a sacrifice, but that’s less fun.
The Day After
We find Mark’s body (it falling was either Mark fucking with us, or just a cool video choice), and Damien(Mark) sees that Colonel isn’t around. He goes to see if Colonel is upset, and finds that the Colonel doesn’t even care that Mark is dead. This gravely upsets Damien(Mark), but he plays it cool and plays his character well. He allows everyone to shift blame around, all the while knowing that the Detective will finger the Colonel as the murderer.
However, Mark knows that getting revenge on only one half of the cheating duo isn’t enough. He needs full revenge. He knows that the Colonel would tell Selene where he was that night, and that she’d show up eventually when he didn’t return home. However, Mark was not expecting Selene to be in-tune with the occult, thus Damien(Mark)’s exclamation, “I just didn’t think that you were the type to become mixed up in all this.” Fortunately for Mark, her powers aren’t too powerful. Mark is able to manipulate the seance in order to get everyone outside and away from Selene. Although Selene isn’t fond of Damien (“I don’t need help, especially from you,” presumably because he took Mark’s side during the affair), Damien(Mark) stays behind, preparing revenge.
Damien(Mark) attempts to kill/possess Selene, but her practice with the occult causes things to go awry. Instead of things being easy, like Damien, Selene fights back, and we see the flashes of lights and Selene’s torn apparition. George closes the door, but Selene is killed and her body is destroyed in Mark’s attempt, as is Damien’s.
Sensing his plan is going awry, Mark then attempts to possess you. He is mildly successful but is unable to complete a full possession. Mark, not fully grasping us, leads us to the room that points to the Colonel being guilty, allowing the Colonel to find us there. This results in a scuffle that Mark hopes results in our death, and it does. As we die, we hear a deep voice say “It’s me.”
At this point, Selene and Damien are no more. Darkiplier is using visions of people we trusted in order to manipulate us. Selene is red. Damien is blue. But who is in the middle, grey? Mark’s body. Mark. “Let me in.” “You have a choice here,” but being given no choice. We are being manipulated. This is Darkiplier. Our body is then brought back, driving the Colonel mad, and our mind/soul is left behind.