Oh, my darlings, let me tell you the proper version. Once, long before the days of Slytherin, there was a witch who lived like a shining star all surrounded by squat, ugly, and sightless Muggles; and so we will call her Pure-As-Stars. Think her up in your mind’s eye. She was the most beautiful creature you can imagine. Pure-As-Stars searched far and low for a proper kind of wizard, and found him. They had a daughter. We will call the daughter Pure-As-Snow. Well, one day Pure-As-Stars decided to take Pure-As-Snow to the market, which was crawling with Muggles, for in those times wizards and witches had no choice but to mix with all kinds of backwards persons. And in the market they passed over all of the rubbish that these creatures pressed on them, and found instead a magical mirror that was far too precious for its filthy surroundings. So, Obliviating the Muggles, they seized the mirror and took it home.
Soon enough, Pure-As-Stars found that the mirror showed them everything they could possibly wish to know. How should we ward the house, so that these creeping and ugly folk living around us might leave us be? asked Pure-As-Stars. And the mirror revealed a way. How might I bend the wild elves to my whims and so make a perfect servant for dear Snow? asked Pure-As-Stars. And the mirror showed her how. Which spells might I use to defeat the hideous hags and giants which torment us? asked Pure-As-Stars. And there, in the mirror, was the answer she desired.
So from far and wide there came ugly, grasping Muggles to look inside Pure-As-Stars’s magic mirror for answers to their trifling problems, and to praise her and marvel at her great beauty, so that after some time when Pure-As-Stars would ask the mirror, Where might I find the most beautiful and extraordinary witch in all the world? the mirror would only show her her own reflection. However, because it was a magic mirror and meant to reveal all the answers one might covet, when Pure-As-Snow came of age, the mirror’s answer to this last question changed, and it began to show Pure-As-Stars’s pretty daughter instead. Startled and angry, Pure-As-Stars began to hate her child, who was nothing like all the ugly creatures around them, and who had instead grown to surpass her lovely mother.
Knowing she was no longer the most exceptional being in the land, Pure-As-Stars took to bed, ill and jealous, and when her husband came and begged for a solution to cure her, Pure-As-Stars said that the only thing to cure her would be to eat the liver of Pure-As-Snow.
Being a proper wizard, canny and clever, her husband hid Pure-As-Snow and killed a wild wolfman for his liver, and when Pure-As-Stars ate it she recovered for a time. But the ugly wolf heart was no permanent cure, and when she looked in the mirror and asked who the loveliest witch in the world was, it again showed her Pure-As-Snow. So again Pure-As-Stars fell ill with hatred, and demanded the stomach of Pure-As-Snow. Her husband went out and killed a troll and delivered its stomach to her, but this paltry offering again failed, and still the mirror showed Pure-As-Snow. So Pure-As-Stars asked for the heart of Pure-As-Snow. And her husband killed a Muggle and brought her the heart, but this would not do either, and Pure-As-Stars resolved to do the task herself. Pure-As-Stars brewed a poison brew and set out to find her daughter with the aid of the mirror, and so to eliminate the competition.
But when she found her, Pure-As-Snow begged to live. “How can I let you live?” said Pure-As-Stars, “When you are not like these rapturous Muggles who do my bidding, but instead seem poised to rise above me?”
"You are too enamored of those lesser than you, and not enough in love with your own kind," said Pure-As-Snow. "You must let me live. I am like you."
Pure-As-Stars said, “But I despise you. You are so much lovelier than I.”
"I am this way because I am your daughter, and the daughter of a worthy father,” cried Pure-As-Snow.
"The mirror has shown me that you have more might and magic than I, being so beautiful," said Pure-As-Stars, "And you will replace me. Why, then, should I let you live?"
"Without me to replace you, there will be nothing but the ugly Muggles to inherit the world. The mirror has shown you what you covet most. Is that not, Mother, the chance to leave behind a child as pure and beautiful as yourself?"
And Pure-As-Stars saw that it was so, but there was chaos and jealousy in her mind which had been left there by all the praise these sightless Muggles had heaped upon her, seeking to gain her favor and her love. And she realized how wrong she was, and drank the poison brew herself, and her death caused a great plague to sweep the land which killed these obsequious beings, a fitting punishment.
And so Pure-As-Snow ruled over the land from that day forward, inviting clever and proper magical persons to live with her, and she locked the mirror away for many generations until one descendant of hers, who we will call Pure-As-Black, who as we know is not a made-up person but a distant ancestor of your father’s, should ship it with him to this country. Pure-As-Black hid the mirror in some secret place, remembering how it had made a mother love Muggles instead of her own perfect child, and charged every mother in our line with telling this story. And the mirror he called Eris, after discord, and those who fall pray to it are the Erised.
So now you know. Yes, Bella darling, every last one of them was beautiful, with the shining dark hair of that distant land they cleansed of Muggles. Yes, Cissy darling, it is a tale of motherhood, the noblest occupation a witch can have. Oh, what’s that, my darling? You think it was silly of her to want to be lovely and perfect? You think that was the real problem? You feel sorry for the Muggles? Oh, Andromeda Lyra Perpetua Black. You do have such fancies.
(Honestly, Cygnus, I worry for her. She does seem to miss the point of the story.)