Now, as you might have imagined, the app has been removed, though people who have it installed can reportedly continue to use it.
As reported by Eurogamer, the app is called Win64e10. It was actually released over the weekend, according to people on Reddit, though buzz didn't really build until days later. According to the site, the app, which cost the equivalent of £7.69, was "hidden on the marketplace as it had not received a Microsoft-approved release." If you knew the app's name, however, you could apparently find it.
Microsoft has now stepped in to pull down the app. People on Reddit report that it still works for those who have it installed, though it has some issues that future patches could have addressed. You can see a YouTube video of how the app works here.
An Xbox One update that came out this year for Xbox One introduced support for the Universal Windows Platform, which basically made it possible for Windows Store apps to run on Xbox One as well. As this example shows, Microsoft is likely to take action against apps of questionable nature, like this one.
GameSpot has contacted Microsoft in an attempt to get more details. We will update this post with anything we hear back.