The theme of the prompt is mystery. Something unknown is afoot and the MC or the reader has to figure it out. So come up with an alien artifact, an arcane spell book, a disappearance or murder and take it from there. Now mystery is closely related to suspense and to help you along here are a couple of definitions (From- https://www.quora.com/What-is-the-difference-between-Mystery-and-Suspense):

A mystery story shows us the major event (murder, robbery, etc.) that has happened first and then we watch as the protagonist(s) solves the mystery of who did it, why they did it, and how they did it. 

A suspense story shows us the major event LAST, and before that we are watching how that event unfolds with twists, turns, and surprises, which keep us in suspense (a state or feeling of excited or anxious uncertainty about what may happen).

But remember this is a prompt not a contest so if you come up with a suspenseful story go with it.

Per usual, post your submission as a child page under the writing prompt page. Post comments and critiques to a particular sub as comments to that page.