The First Evil from Buffy the Vampire Slayer. Think about it, it’s only been shown as taking over the form of dead people (Locke, Alex), that guy on the beach (possibly, we don’t know if he died or not) notwithstanding.
Only difference is that it’s corporeal (able to touch things.) I’m half expecting Sawyer to don a red robe and get scars on his eyelids next episode.
The credits for the finale will have “STORY CREATED BY JOSS WHEDON” at the end, I’m sure of it. Sarah Michelle Gellar guest appearance, too.