Mirror dimension, hell is nothing compared to this place. To make the experience even more immersive, you can add your phobias or past trauma events in your persona

You sleep the day before, and you experiencing weird dream about moon, sun, and the universe. It has something to do with your own fears, now you wake up in the Mirror Dimension, you have to find a way to get out, either fight every living here, or using your brain to an extent, or its the end for you.