Fawkes x Fiendfyre Phoenix. Bonus points for QM and AD standing around uncomfortably and carefully not watching, since it would be too awkward for either to be the first to say anything or resume their not-the-kind-of-climactic-they-were-expecting final battle.

"One strike of my wings on your breast," shrieked the fiend, "and you would die in agony."

"Or perhaps," shrieked Fawkes, "we would short each other out in the most exquisite agony ever created!"

They circled, double helixing upward until Hogwarts was concealed by mist.

“Our masters are dead. You have just an hour until the spell that made you decays, I an hour until I have to depart."

"I should be kamikazeing myself in crimson-flamed fury.”

"But as long as I, a remnant of your enemy, live...”

The fyrebird lunged, and Fawkes broke into a steep dash upward.

"No phoenix is allowed to enter space, just as all Atlantean magic ends at the stratosphere. But even across its distance, we hear the courage of aliens."

"Ah, and I feel the most lightspeed hate!”

The plumed thunderbolt and her flaming, black-veined pursuer broke through a cloud into a layer of the sky where stray phoenixes thronged.

“See? We're by no means antitheses."

"There are so many! Where do your kind come from?”

"This is the Phoenix Gate."

It was floating golden oval moving through the sky, towards them. Never in his seven hour life had the dark firebird felt such instant loathing, such intent to destroy. He froze. "What is it?"

"The Atlanteans long searched for the best government. Cautious of despotism, they decided it was important to put up safeguards...so that those with the right urges would always have ways to be heard. I was born from it, knowing nothing but that, my powers, and my summoner's will."

"I was made to break this." 

But already the egg had soared over them and was shrinking, arcing into the distance.

Voldemort's phoenix barely thought before racing after the egg, streaming a balefire backwash, and now it was Fawkes's turn to chase.

Fawkes said, “We have six minutes to live. If you rush you might catch up with it. Instead, I give you the choice to rape me to death right now."

"I was summoned with hate of your mage in mind. But the spell I am was invented to destroy that egg.”

In response, only foxy shrieks.

Deafened by the thunder of his own flight, torn between two aims and the world closing down as the spell of his body wavered, burning lower and lower, Balefire stole a look back.

Fawkes was in the pallor of her beauty.

His heart inflamed with short-term need to destroy, and he wheeled straight around. The veins of black fire that filled him shot out in tentacles.

It was all faster than Fawkes could anticipate. One minute she was hurtling forward, and the next a shadow blotted her vision, searing black tentacles had pinned her wings, and then a fiend-cousin like a dying sun rammed through and incinerated her, then touched by her last tears itself combusted, a spectacle not seen since Atlantean times.

/r/HPMORkinkmeme Thread