If you accidentally -- [ Stephen's whispering too before he realises he's doing it. And then he's shaking his head, bringing his arm up in an arc as the space before them seemingly cracks into pieces of glass: layers upon layers of chunks like icicles in a cave, only the sunlight filtering into the sanctum diffuses and turns into washes of rainbow colours. ]

The Sanctum is going to be fine.

[ He hopes to reassure her both in the steadiness of his voice and in what is, with luck, a calming expression. ] I'll show you. [ Then he's stepping into the broken glass, fully expecting her to follow, and though the world seems to erupt for a split-second into shimmering lights, everything becomes clear after.

And the world here looks exactly the same as reality, save for the random cuts in the environment, turning everything into chunks and pieces. In the crudest definition possible, it's as if everything around them is only a reflection on a broken mirror.

Stephen, however, looks the same. And he turns to face Mantis with a quick spell forming a ball of energy in his palm. ]

This is the mirror dimension. [ He bounces it up and down like a baseball, then reels his arm back to throw it directly at a window. Though the ball explodes, the window is unharmed -- but does, as everything else in the mirror dimension does, still hold its distorted shape. ] When you're in here, you can't affect the real world.

It's the safest space to practise anything.