This, unlike the light, does him some harm. The Sword of Gryffindor, legendary blade of the bravest founder, forged by goblin master smiths in the fire that is the blood of the earth, treasure of Hogwarts since his death, thrice stolen and thrice returned, shining with rubies and silver, clonks Harry Potter on the noggin.

"Oi! Over here, you scaly git!" yells Ron, just before he whirls behind a pillar. 

The basilisk's own vast muscles propel it headfirst into the stone with a terrible CRUNCH. The car-sized head withdraws, stunned but furious, to reveal a crack running up the pillar.

Nothing they're doing is hitting hard enough. Her spells aren't going to bring it down before she runs out, her sword isn't going to bring it down before it gets lucky, Ron's courage and cleverness might bring it down but might bring the ceiling down on top of them as well. 

When Harry yanks the sword out of the hat, it's obvious that this is the phoenix's plan and their best shot. Maybe it's matched to the basilisk's vulnerability*. Maybe it's just extremely magic. But it's the right sword for the job and Harry is the right person to wield it.

"Inflict Moderate Wounds. You've got this, Harry!"

*Author's note: Avistani natural history doesn't have vuln and DR as separate concepts. They know there are monsters that are best attacked with particular materials but not that there are two different ways for that to work.

He has no idea how to use a sword, but he knows which end is the sharp bit! Time to stab!

The basilisk hiss-shrieks as a gash opens up on its side and that gash then gets a sword shoved in it. But now it knows exactly where at least one enemy is. Time to BITE.

And Harry lifts his sword and braces his feet with the steadiness of an infantry line facing down a cavalry charge, and steps forward at just the right moment as the basilisk's fangs come down and stabs off-center through the roof of its mouth.

The basilisk crashes to the floor, wounded beyond what even its vast health can bear. With the last of its strength and a voice distorted by the blade through its skull, it hisses, "Fffriend?"

"I didn't expect you to be so useless. Losing to two kids and a, whatever she is? Pathetic."

"I'm sorry it had to be this way," Samora murmurs as the great serpent's final breath shudders and stills. "Sarenrae, who welcomes all souls, may your lawyers be as ever wise and learned. Erastil, watch over this creature, who died protecting its home."

And then she turns to the Dark Wizard who spent the whole battle standing and watching as though indifferent to the outcome. "Last chance."

"Ha! I don't need the basilisk, and there's nothing you can do to me. I'm immortal." And indeed, while the Holy Smites clearly hurt, he has no visible wounds.

Harry is looking back and forth between Riddle, Ginny, and the diary lying on the floor.

Ron, who has snuck halfway around the edge of the Chamber unnoticed, darts out from behind a pillar, scoops up the diary, and throws it to Harry--

Who catches it one handed and impales it on the tip of Gryffindor's blood- and venom-dripping sword.

AAAAAAAA

AAAAAAAA

AAAAAAAA

AAAAAAAA

AAAAAAAA

Ron runs to take his sister's hand. "Ginny?"

". . . Bwuh . . . Where am I? Ron, are you okay? I--I--" Ginny rolls to face the wall and starts sobbing quietly into her hands.

Hug. Hug hug hug.

Cling.

"I t-tried to tell you at b-breakfast, that it was me--I'm going to be expelled--what's Mum going to say--"

"You're not gonna be expelled. It was Riddle, he made you do all that stuff, and he's dead now. And the snake's dead too, you're safe, everything is gonna be okay. We just need to get out of here." Which they are definitely going to be able to do without a giant snake to ride, somehow.

"At dawn tomorrow I'll be able to prepare Angelic Aspect and fly us out of here, if we don't find a better way by then. Does anyone need healing or a drink of water in the meantime?"

Nobody got hit. Ginny will accept the offered waterskin after Ron gives her a look about it.

I can carry everyone if you carry each other 

There is no need to wait

It is always dawn somewhere 

The only thing that isn't awesome about phoenixes is that they tend to hang out on remote mountains instead of going where the trouble is, and this one isn't doing that. What an excellent friend.

"Thank you!" She follows Fawkes back towards the end of the giant pipe.