Delphi would have screamed, but she couldn't make a sound. Where there should have been a back to Quirrell's head, there was a face, the most terrible face Delphi had ever seen. It was chalk white with glaring red eyes and slits for nostrils, like a snake.
"Delphi Tonks…" it whispered.
Delphi tried to take a step backward but her legs wouldn't move.
"See what I have become?" the face said. "Mere shadow and vapor….I have form only when I can share another's body…but there have always been those willing to let me into their hearts and minds…Unicorn blood has strengthened me, these past weeks…you saw faithful Quirrell drinking it for me in the forest…and once I have the Elixir of Life, I will be able to create a body of my own….Now…why don't you give me that Stone in your pocket?"
So he knew. The feeling suddenly surged back into Delphi's legs. She stumbled backward.
"Don't be a fool," snarled the face. "Better save your own life and join me…You are mine, Delphi Tonks. You always were. Did you truly believe yourself to be the child of those mediocrities?"
"I don't understand," Delphi replied.
And then, her scar burst with pain...
A dark-haired woman who looked so similar to Andromeda Tonks and yet so different cradled a tiny newborn infant as the snakelike man watched.
"Delphini Bellatrix Slytherin."
"She's your daughter, my lord."
"She is irrelevant. The prophecy must be averted."
"You can't kill her. Not our Delphi."
"You have been a good servant. Stand aside and I shall allow you to join me again."
"Avada Kedavra!"
"You liar!" Delphi shrieked as memories assaulted her. It wasn't possible. It could not be possible. She was Delphi Tonks. Daughter of Ted and Andromeda. Sister of Dora. Best friend of Hannah and Justin. The best at magic in her year. Hufflepuff. She could not be Voldemort's daughter.
"Understand the truth," Voldemort hissed, "You are mine forever. Bellatrix needn't have died…she was trying to protect you…Now give me the Stone, unless you want her to have died in vain. "
"Never!"
Delphi sprang toward the flame door, but Voldemort screamed "SEIZE HER!" and the next second, Delphi felt Quirrell's hand close on her wrist. Another needle-sharp pain seared across Delphi's scar; her head felt as though it was about to split in two; she yelled, struggling with all her might, and to her surprise, Quirrell let go of her. The pain in her head lessened — she looked around wildly to see where Quirrell had gone, and saw him hunched in pain, looking at his fingers — they were blistering before his eyes.
"Seize her! SEIZE HER!" shrieked Voldemort again, and Quirrell lunged, knocking Delphi clean off her feet landing on top of her, both hands around Delphi's neck — Delphi's scar was almost blinding her with pain, yet she could see Quirrell howling in agony.
"Master, I cannot hold her," Quirrell protested, "— my hands — my hands!"
And Quirrell, though pinning Delphi to the ground with his knees, let go of her neck and stared, bewildered, at his own palms — Delphi could see they looked burned, raw, red, and shiny.
"Then kill her, fool, and be done!" screeched Voldemort, clearly not competing for father of the year.
Quirrell raised his hand to perform a deadly curse, but Delphi, by instinct, reached up and grabbed Quirrell's face —
"AAAARGH!"
Quirrell rolled off her, his face blistering, too, and then Delphi knew: Quirrell couldn't touch her bare skin, not without suffering terrible pain — Well, the man had just tried to kill her. Delphi figured she may as well pay him back in kind.
Delphi jumped to her feet, and placed her hands on Quirrel's face, being sure to scrape her thumbs along his eyes, burning them and blinding him. Quirrell screamed and pushed Delphi away. Delphi's scar burned, and she could only hear Quirrell's terrible shrieks and Voldemort's yells of, "KILL HER! KILL HER!" and other voices, maybe in Delphi's own head, crying, "Delphi! Delphi!"
Delphi ran in again, scraping her hands along Quirrel's face. The man shrieked in agony and the girl applied as much pressure as she could. Maybe she could kill Voldemort as well as Quirrel. That would serve him right for lying to her.
Or worse, for telling her the truth.
The smiling face of Albus Dumbledore swam into view above her. Before even registering where she was, Delphi realised the old man knew. He had to know.
Enraged, the girl leapt out of her bed and slammed herself into Dumbledore.
"You liar!" Delphi shrieked, "You fucking liar!" Ted and Andromeda didn't like swearing, but she didn't care at the moment.
With a twitch of his wand, Delphi found herself rising into the air, before gently being dropped onto the bed.
"Delphi," the headmaster said softly, "I am so sorry for deceiving you."
"Sorry's not good enough," Delphi replied. She folded her arms, only to realise how immature that must have looked.
"Nobody was aware that Voldemort had a daughter," Dumbledore explained, "The decision was made for your protection, not only from your true father and his supporters, but from some of the general public, whose goodwill I feared may have faded if they discovered the truth about the parentage of the Girl Who Lived."
"Get out," Delphi snapped.
"Delphi, please allow me to explain-"
"I said GO!" the dark-haired girl thundered. Dumbledore sighed, turned on his heel and walked away.
When Justin and Hannah came in, Delphi opened her mouth to speak, only for the sobs to escape involuntarily.
Her two friends ran towards her, enveloping the crying girl in a hug.
"It's OK," Hannah whispered, "You're home now."
I know this chapter was meant to be COS, but the Quirrell battle seemed like a more appropriate place for the reveal of Delphi's parentage.
As you can probably tell, Delphi is noticeably more ruthless than Harry, and perhaps has something of Voldemort and Bellatrix's sadistic streaks. She was actively trying to kill Quirrell. However, she's also more sensitive and prone to becoming emotionally overwhelmed.
To clarify, this will not be a Dumbledore bashing fic. The two will have a generally good relationship, if perhaps a bit more untrusting than Dumbledore/Harry. She's just very angry with him at the moment.