He knows how to use the power of the skeleton key. When we get it we don't think like Mercer, we see it as just an unbreakable lock pick because the Dovahkiin isn't that intelligent. It is more than a key but kind of like a door way to unimaginable power, probably why it is shaped like a key because it unlocks abilities you're not meant to. He probably opened it with some crazy technical idea that the very idea is insane, he unlocked the door because there was no door and since there was no door, the door opened because to be closed it'd be a door and it is no longer a door and then the skeevers will evolve into intelligent master race and rule over land using Men and Mer alike as slaves.