How do you write a story about an artifact?

The same way you write any other story -- plant your butt in a chair and get started! Make an outline listing everything you know about the artifact. Then, change each line into proper sentences and string them together into some sort of order -- any order that makes sense to you. There's your story.

If you mean a fiction story, you'd already have the idea if you're meant to write that story. If you can't think of an idea, you don't really want to write the story.

Here are some links.