I am GMing a one-shot campaign for my brother and a couple sisters and I have the start of an idea but I'm drawing a blank at parts. What I have so far is they start in a town that's in a panic. Each day someone new falls into a strange sleep, unwakable, unending (thinking sleeping beauty or snow white). Originally I was thinking of having a Dragon be the cause of this, but I couldn't figure out how a dragon that is fightable for a group of level 7s would be capable of that, or why it would want to do so. Though I could just make up a new dragon from scratch, I am new to GMing and my homebrewed monsters are often overpowered or underpowered, and very unbalanced.
So what I am asking here is for ideas of what could be the cause of the town falling asleep. For all I know I may end up just trying to make the dragon, but I figured it didn't hurt to ask and see what ideas others might be able to offer.