Honestly, I don't believe Celine and Damien in the end were Celine and Damien at all. They were puppets. (If it truly was them, they were controlled by Mark.)

When the Colonel killed Mark (And I do believe he killed Mark. If they got into a spat like the Colonel and the detective... well look at what happened to both us and the detective.) It left Mark as a vengeful spirit. The house was always a "den of evil", a place where the border between ours and the spirit world is thin and blurred. Mark is now trapped there, and he is angry.

(Everytime someone says murder, it's Mark's spirit lashing out in anger.)

When Celine and Damien are left alone, Celine, being a seer, acts as a catalysts between the two worlds, allowing Mark to reach through, kill them both, and whisk them away. He now has two pieces of the puzzle he needs to get his revenge.(I went back to watch the last episode and I realized that Celine told them that her session could not be interrupted. Guess what happened? It was interrupted. And at the end of the episode, when everyone runs to the house Celine is both red and blue, and Damien was no where in sight. It's possible that Mark tried to use Celine's body and failed.)

Knowing the Colonel is looking for us, he uses Celine's pleas to lure us into the detective's room. He wants the Colonel to see the detective's notes, to see his accusations, he wants the Colonel to confront the dectective, because he knows Abe is aware of the affair, of which was possibly the reason Mark was killed. He wants the Colonel and the detective to kill each other. What he possibly didn't factor in (or maybe it was his plan all along) is that the detective would survive, and we would die in both of their stead. Regardless, a body is a body none the same.

We are now dead. A spirit in the house, the same as Mark, whom is the first person we see.

"It's not fair, is it?" He goads, his face shifting to something less human. He wants us to be angry too, he wants us to want to leave this hellhole.

Enter Celine and Damien. Two people we know, one of them whom we trust. Mark uses them as puppets to talk to us, telling us that we are dead, but that this death is not the end. Everything that is said henceforth is used to either bait, or misdirect you. (At this point I would like to point out the real Mark's comments about Darkiplier: "He is a social manipulator. He leads you into this false sense of security, and he wants you to trust him, because he wants to take advantage of you.") "You can't do this alone, you need me. Trust us, trust me. Let me in, and I promise I will fix this.” Sound familiar? The vengeful spirit manipulates us into a false sense of security, and gives us a choice, which is in fact, no choice at all. "This is the only way you can escape." Our death, isn't fair, is it? Of course, we would chose life. So we let him in. Now the spirit has what he wants, our body, but with us in it. We awake, jarred, but alive to find the Colonel, now mad with the death of all of his friends. We take the now discarded cane, and Dark awakes. He takes Damien's form, (The real Mark has mentioned Dark can do this, and it makes sense. Damien is suave, someone that can be trusted) and locks us in the mirror. His plan is complete.

We are now a burden.

We are no longer needed.

And we are now trapped in this hellish house for eternity.

Now, to go a little deeper into this lore, I don't think it's farfetched to think that it wasn't ever Mark at all, but a spirit/demon/evil-thing-what-have-you that used his death as a means to put it's own plans of escape into action. Or, possibly, that the evil of the house corrupted Mark. There's a lot of open ended questions here that I'm sure Mark will answer later, but I find the whole "Dark is three souls shoved into one body on a crusade to exact revenge against Mark for their deaths" to be a little... Odd.