It's an alternative dimension that mirrors our own, comprised of fractals and may be distorted, in a very Matrix sort of way. What occurs in the Mirror Dimension has no effect on what happens in ours, and is often used for training and teaching in my universe. I'll show you, if you come. The only way to access it, as far as I know, is with my sorcery. This way, if an accident does happen, then our world will still be protected, though we'd have to deal with them.
We plan to build the containment unit inside the Mirror Dimension, in the basement of the Sanctorum, but if you can find us an abandoned something, somewhere, that would be very helpful. But you'll need to find it quickly, because time is of the essence.
Vision has elected to stand guard of them. Being an android, he is not distracted by physical necessities such as food and sleep, which might cause him to make mistakes. Charlie Weasley has also volunteered, who is accustomed to dealing with magical creatures of his world, as well as Alphonse, who I understand works with elemental magic.