When Ilmare has finished laying her magic, Mairon looks into the mirror he has created. He claims, to himself and to others, that it's to see whether it works properly; in truth, he is curious.
The reflection is of himself in his forge, surrounded by things of beauty, things he has created that will make the world better.
Mairon smiles and sends the mirror on its way.
The next time he sees the mirror, he isn't Mairon. He is in Middle-Earth, his name is Sauron, he is the lieutenant of Melkor, and he has done things that to Mairon would be unthinkable.
He knows what he will see: himself, with the world at his feet. Second only to his lord and lover.
But Sauron looks in the mirror anyway.
He sees a man, a few inches shorter than himself but with the same slim build, same blonde wavy hair, same dark eyes. He's smiling in the mirror, soft and gentle, and Valinor is in the background.
He sees Mairon.
The mirror is too well-crafted for Sauron to break.