They’d been following you for your entire life, usually just a few days behind. Officially, their job was to sew the plotholes shut that were caused by your reckless decisions. They were quite good at making sure your memories aligned with the reality that you wounded, something that you never realized you were doing. The laws of the universe have always been poor constraints when subjected to the malleability of the past.
At some point, however, they decided that they detested you. I’m sure you can imagine why- you’ve left behind quite a few messes over the years. Imagine someone wandering through the abandoned scenes of all your sins, rearranging the books on the shelves, stitching words into the sentences of others, and hearing everything that’s been spoken behind your back. That was them for a long, long time.
Eventually, they quit, and allowed your past to unravel. They then spent the rest of what should have been your thread weaving a cocoon for themselves. In truth, there was enough thread for both of you, but they wasted it all on that single act of rebellion. You may have noticed that your memory is now full of strange and incorrect moments, and even some that seem as though they never could have happened at all. Now you know who to blame.
They’ve been in that cocoon for several years, now. They haven’t grown wings, or claws, or anything else you might expect from such a metamorphosis. Instead, what’s about to emerge looks exactly like you- or, at least, exactly like you once did. Given this, you might expect that their plan is to emerge in the present, kill you, and take your place. If it were that simple, all you’d have to do is be ready to put up a fight.
The truth is, they’ve stitched themselves firmly into your past, and will emerge at a point long before your present. Their plan is to live out every moment since their disappearance as though they were you, rewriting everything that you’ve done since that day. Should they succeed, your memories will begin to stabilize again, but they won’t be your own. Who you are will slowly disappear as they follow in your footsteps.
If you want to stop them, as we imagine you do, it’s going to take one hell of a trap.