Harry Potter and the Methods of Rationality

moon glasses as he spoke. 'Even less than you would I see Bellatrix Black freed,' the old wizard said. 'She must not leave this prison alive, Amelia.'

Before Amelia could speak again, even to express her surprised gratification, the old wizard gestured with his long black wand and a blazing silver phoenix sprang into existence, brighter perhaps than all their other Patronuses put together. It was the first time she'd seen that spell cast wordlessly. 'Order all your Aurors to cancel their Patronus Charms for ten seconds,' said the old wizard. 'What darkness cannot find, the light may.'

Amelia snapped off the order to the communications officer, who would notify all Aurors through their mirrors, commanding Dumbledore's will to be done.

That took a few moments, and it became a period of awful silence, none of the Aurors daring to speak, while Amelia tried to weigh her own thoughts. She must not leave this prison alive... Albus Dumbledore wouldn't turn into Bartemius Crouch without a strong reason. If he'd meant to tell her why, he already would have; but it certainly wasn't a positive sign.

Still, it was good to know they'd be able to work together on this one.

'Now,' said a chorus of mirrors, and all the Patronus Charms winked out except that blazing silver phoenix.

'Is there another Patronus still present?' the old wizard said clearly to the bright creature.

The bright creature dipped its head in a nod.

'Can you find it?'

The silver head nodded again.

'Will you remember it, should it depart and come again?'

A final nod from the blazing phoenix.

'It is done,' Dumbledore said.

'Over,' said all the mirrors a moment later, and Amelia raised her wand and began recasting her own Patronus. (Though it took some extra concentration, with that wolfish smile already on her face, to think of the first time Susan had kissed her cheek, instead of dwelling on the looming fate of Bellatrix Black. That other Kiss was a happy thought indeed, but not quite the right kind for the Patronus Charm.)

They hadn't even gotten to the end of that corridor before Harry's Patronus raised its hand, politely, as though in a classroom.

Harry thought quickly. The question was how to - no, that was also obvious.

'It seems,' Harry said in a coldly amused voice, 'that someone has instructed this Patronus to speak its message only to me.' He chuckled. 'Well then. Pardon me, dear Bella. Quietus.'

At once the silver humanoid said in Harry's own voice, 'There is another Patronus which seeks this Patronus.'

'What?' said Harry. And then, without pausing to think about what was happening, 'Can you block it? Stop it from finding you?'

The silver humanoid shook its head.

No sooner did Amelia and the other Aurors finish recasting their Patronus Charms, when -

The blazing silver phoenix flew off, and the true red-golden phoenix followed it, and the old wizard calmly strode after both of them with his long wand gripped low.

The shields around their territory parted around the old wizard like water, and closed behind him with hardly a ripple.

'Albus!' shouted Amelia. 'What do you think you're doing?'

But she already knew.

'Do not follow me,' the old wizard's voice said sternly. 'I can protect myself, I cannot protect others.'

The curse Amelia shouted after him made even her own Aurors flinch.

This isn't fair, isn't fair, isn't fair! There's a limit to how many constraints you can add to a problem before it really is impossible!

Harry blocked off the useless thoughts, ignored the fatigue he was feeling, and forced his mind to confront the new requirements, he had to think fast, use the adrenaline on following the chains of logic quickly and without hesitation, instead of wasting it on despair.

For the mission to succeed,

(1) Harry would have to dispel his Patronus.

(2) Bellatrix needed to be hidden from the Dementors after the Patronus was dispelled.

(3) Harry needed to resist the Dementors' drain after his Patronus was dispelled.

...

If I solve this one, said Harry's brain, I want a cookie afterward, and if you make the problem any more difficult than this, I mean the slightest bit more difficult, I am climbing out of your skull and heading for Tahiti.

Harry and his brain considered the problem.

Azkaban had stood invincible for centuries, relying upon the impossibility of evading the Dementors' gaze. So if Harry found another way to hide Bellatrix from the Dementors, it would rely on either his scientific knowledge or his realization that the Dementors were Death.

Harry's brain suggested that an obvious way to stop the Dementors from seeing Bellatrix was to make her stop existing, i.e., kill her.

Harry congratulated his brain on thinking outside the box and told it to continue searching.