The Others know about Mr. X by Ricardus Alpert

Most of us naturally assumed The Others had no idea there was another entity on the island, and that Jacob had an enemy.

But they do know of him. Or at least, some of them do.

I believe Bram and Ilana represent a different faction of The Others, like Ben's off island people. It's no coincidence that Bram repeated Ben's "We're The Good Guys" claim.

Merely seconds before he did, Bram told Frank of X. Here's what he had to say:

Bram: We need to show it somebody, so they'll know who they're up against.
Frank: What are they up against?
Bram: Something a hell of a lot scarier than what's in this box.

But it's hinted at even before S5 started. In "The Cost Of Living" we see Colleen's funeral, and we get a glimpse of how The Others treat their dead. They burn their bodies and send them out to the sea.

Why? Because The Others know what happens to bodies that are left unburied on the island. Something takes their shapes.

But If the Others had virtually no contact with Jacob, who could have told them about X?

Richard did. I believe that at that point, Richard believed X to be finished, but he instructed his people to burn the bodies, and it became a part of their traditions. I think X became something of a "Bogeyman" to The Others, over the years. A dark force on the island that can take the shape of their dead.

I suspect they're going to be terrified of him in S6.