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The process of a soul to become a demon, once the soul chose the sin conenant they want to join, is very fast, some says on a blink of an eye even. Many of the newborn demons never stop to think about what just happened and just goes with that, but the few who stops and takes moment to process everything, they get a jarring realization: they are dead, they went to Hell and the demon in the mirror is their reflection, their new body and life, they are demons now and there isn't nothing they can do to change back. With that on their mind, they eventually, and reluctantly, accepts it and goes on with their new life in Hell.

And also there are those who looked at the mirror with horror, because they may chose the wrong covenant and didn't like their new look, they may be able to change pacts but has to wait around five years to do so.