I was in a world with a river full of strange stones that would grant people dark mutant powers if they touched one. Each person's power was tied directly to the stone that gave it to them, and I had to destroy their stone in order to defeat each mutant.
An evil queen desired to become the most powerful mutant and control the world, and sought out the most powerful stone for herself. But in the end, I was able to use my crossbow to destroy her stone before she was able to touch it, preventing her dark reign.