In my first story, Lights Out, Pinkie Pie was attacked by an unknown unicorn. After the attack, she doesn't seem right...
After visiting Pinkie Pie in the hospital, Twilight and friends find out what's wrong, and what happens next is something horrifying. When Twilight and friends go back to the Twilight's castle (Pinkie Pie ran out of the hospital before they left), they discuss searching for Pinkie before deciding that it was too late to search. They figured they'd fall asleep during the search, so they went to sleep anyway.
They didn't realize that going to bed for the night would be a big mistake.