Rainbow Dash finds a stone while out flying, but it explodes when she touches it. What was it and why did it explode?

Based on a prompt by OCisbestpony

Rarity's horn has been known to do some very strange things without Rarity's permission. When she's called on a friendship mission, she doesn't know what to expect. Her horn, on the other hoof, has plans of its own.

Everypony thought the windigos were as good as gone. Then the Cutie Map calls the Mane 6 to the top of a mountain, and they find out otherwise... Can Twilight and her friends stop the dastardly plot before a villain can allow the windigos to return?

Set near the end of Season 6 and includes occasional references to events in Season 7.

Preread by:
CapriciousCapricorn
indianaiam
William