He closes the door. This is much easier than opening it from the outside.

"Further down that tunnel is the location of Ruin's prison, at least as much as it has a physical location. I don't trust oral tradition, especially since Ruin has shown the ability to alter or fake any records not written in metal. As the only other person likely to live three hundred years, you're now my only credible backup plan other than saving the world myself. Congratulations."