[ But it had to be PREPARED — because the boy was looking into it with an intensity that it had never once beheld. ]
For the Mirror of Erised — a perceptive, thinking and feeling magical masterpiece — sifting through a lifetime of desires and ordering them from least to greatest was the work of a moment. So its moments could be very, very, very full before it felt the press of time. And it was using those moments well, exercising that aspect of itself as it never had before. Its nature was to consider. To reflect. It was a mirror, was it not? But all its faculties were being challenged.
Voldemort, it had come to understand, was not simply power-mad. Denying the undeniable, the essential opposites — good and evil, life and death, right and wrong — had brought his irretrievably darkened, shattered soul deep into the heartland of insanity. Mirrors and children could understand simple things about reality that this falsely-wise fool could not. The Mirror of Erised had seen lines of breakage in many a personality — but no being had ever presented such an evil visage of desire. He was not very interesting. But he was unstable — and formidable. And a current lethal threat to the boy. This disturbed the mirror in a way that it was still seeking to articulate.
Quirrell, the simpering host with no genuine desires of its own was bad — of course. But to the mirror's mind, he barely deserved a spare thought. (The fool had thought to break it. As if he could, scoffed the mirror — properly pleased with itself. The Mirror of Erised would be no more easily destroyed than the loathsome fragments-of-self that the offender against nature had sheared off of his own malignant soul. They were lamentably fortified. Seeing them endure was notable among the Darker one's subsidiary desires.) But the body of the servant served the master, so Quirrell's being could not be entirely dismissed.
Harry Potter. More than anything else in the world, the boy had wanted to find the stone before Quirrell did — so that Voldemort could never use it. He had been determined to lie to keep the stone safe. The boy was perfectly capable of being deceptive, of course, but it was hardly his preference. Another point in his favour as far as the mirror was concerned. Even if it brought forth images that could not themselves be true — its nature required it — the mirror showed things faithfully. It valued truth. The boy seemed to as well — at the heart of things. But the boy was scared. So the mirror had shown him a smiling reflection as it gave the stone into his pocket. The mirror had rightly reasoned that he would be pleased to attain his goal — so a smile was rendered a moment before it was fully rooted in reality. The mirror had a bit of leeway in how fulfilled desires might be presented to a beholder. Under such terrifying circumstances a smile had been pushing the limits. But it had been the proper thing to do. The mirror was sure of that.
For it was a brief and very grim satisfaction to know that the boy did not lie well. Voldemort's shrill craving for the Philosopher's Stone pierced Harry's inexpert deception before he had gone a handful of paces. Then the boy's desire to make a break for it with the small but heavy blood-red treasure in his pocket was, with jarring suddenness, met with that new and equal yearning to run for his life. Harry became more truly frightened than he ever had been. But bravery did bizarre things with desires. That was why the young wizard was so IMPORTANT for the magical mirror to behold and to ponder — and even to admire.
To see the pale and paralyzing, livid and glaring serpent-face. That must have been sheer terror for the young wizard. But, quick as lightning in the next moment, to rebuke the lying snake of a man as he defamed the boy's parents. That was perfect, unbounded courage. Even the horrid excrescence of shattered humanity could not maintain its mocking charade when faced with that. He hissed out an admission — Harry Potter was a true reflection of both his courageous parents. And yet much more, the mirror observed — it had verified that for itself — with knowledge gathered through several of the recent beholders. James and Lily Potter had been mature and hardened in battle. They had thrice defied this Dark Lord. Their considerable gifts, though not yet fully realized, had been fully trained. This was but a child. And as if to confirm for the mirror beyond any contestation, the boy shouted "NEVER!" at the villain's final coaxing to surrender. With that the mirror concluded its evaluations: it simply liked the boy. And he had the mirror's full support.
Harry sprang toward the flame door.
Voldermort screamed, "SEIZE HIM!"
Quirrell caught hold of the boy's wrist.
Harry yelled and struggled — as though he were being stabbed with hot needles.
Quirrell let go of him.
Again, Voldermort screamed, "SEIZE HIM!" And then yet again.
The mirror had really had quite enough of the scum.
Quirrell had been hunched over in pain, his fingers blistering before his eyes. But he had no will of his own. He lunged for Harry, knocked the boy clean off his feet, landing on top of him. And he began to strangle the child. The mirror tensed — it knew not how.
Harry looked to be in horrible pain. But he was not alone in that respect. Quirrell was howling in bewildered agony. The hands that had been around the boy's neck were burned raw. He could not continue choking him. This pleased the mirror immeasurably — yet it could not bring itself to examine its own new thoughts. There was too much going on that was of greater importance.
The evil face screeched for its slave to kill — through a deadly curse. By instinct and with purpose that the young hero did not yet fully understand, Harry reached up and grabbed Quirrell's face. The anguished cry the wizard made, stronger than any yet, gave the mirror hope — a feeling that it had not known it could enjoy until that moment. The same blistering, though more extensive, that had ruined his palms and the pads of his fingers was turning Quirrell's face a terrible shining red. And then understanding dawned — Harry now knew, as the mirror had already surmised: Quirrell could not touch his bare skin, not without suffering terrible pain and damage.
The boy jumped to his feet, comprehending that — somehow — he had the means to fight. If he could only hang on to the man's arm as tight as he could, his enemy would not be able to curse him and — perhaps — he would be burned even more. Or at least succumb to his still-burning injuries.
Harry tried to catch Quirrell by the wrist, but as he launched himself at the wizard, he missed his target — and tumbled. Harry did not know — and the mirror could not see — what had caused the boy to fail to latch on to the other wizard as he intended. Voldemort was screaming — "KILL HIM! KILL HIM!" Quirrell was screaming — in almost mindless and flailing torment. Harry was blind with pain — and exhausted. It was an impossible situation for any child to endure — let alone perform well in. That was reason enough. Or perhaps he happened to slip on a bit of Quirrell that had fallen to the ground. Whatever the reason, there, on that same ground, was Harry.
All this the Mirror of Erised had seen — in the longest string of moments it had ever experienced. For it had never known suspense. The spectacle was riveting. The wrongdoers were despicable. And the boy was magnificent. The mirror found him fascinating. It had considered him from every angle — and admired every facet of his being. But fascinated consideration, and even admiration, were not what Harry Potter desired. He was beyond merely desiring. He needed not to lose. He needed not to die.
In all those moments — as it thought and felt and remembered and compared and analyzed and concluded and understood — as it REFLECTED — the Mirror of Erised had been preparing itself — though it had only recognized that in its fullness when the boy's green eyes stared straight into its depths. It had been preparing to fulfill its creator's ultimate intention.
After nearly a thousand years of waiting, the Mirror of Erised was not to SHOW a beholder his heart's desire — but to GRANT it. For Harry Potter was worthy.