Notes: I’ve chosen to Potterize Mythology a bit here. I have a sensible canon explanation for how a pureblood witch made herself truly immortal, but to tell Riddle about it would AU the world. So she holds her silence.
In short, my theory runs that Horcruxes will stop your soul from dieing, but not your mind. The Philosopher’s Stone will stop your mind from aging, but will not save your soul from attack. She has made Horcruxes out of non-destroyable objects (the mountains themselves) using the deaths of people who she has fed the Elixer of Life, making herself so immortal that she cannot even cause her own death.
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I am immortal. Tom Riddle repeated it, an incantation against fear, as he stood on the hilltop, mist beading his eyelashes as he stared resolutely east through the mist. I cannot be killed.
The mist glowed, amorphous and swirling as thoughts in a Pensieve, pale blue. A breeze stirred across his dampened cheek and the air lightened. The sun labored upward, pregnant with the coming day’s heat, and Tom awaited her.
The first edge of her light spewed over the veiled horizon, flushing the mist red. Blood has no power over me.
The laughter that sliced the mist was not comforting, nor confidence inspiring. It was bitter, mocking, something he would expect from his own lips. Not friendly at all. He found that the space between his shoulder blades itched with the expectation of a curse, and he held himself still.
“So the old woman was right,” he said.
“Her life was but an eye blink.” The voice was feminine, crisp, cold, a winter’s gust in midsummer. Tom half expected the mist to blow away under the force of it. “A long, torturous eye blink of a running sun and swift-breathing years.”
She stepped into his line of vision, circling from his left, stepping in until she towered before him. Her smile was her laugh made visible, her face pale and still. She looked no older than he.
He bowed his head, with respect, or a mockery of it. “Who are you?”
“I am forgotten.” There was bitterness there.
“It was rather difficult to find someone who could tell me of you,” he said, and there was no doubting the mockery.
She went still, silhouetted in the bloodied mist, tall and dark and terrible. “I could kill you with a thought,” she said, low, contemptuous.
Tom held himself still - like a mouse before a snake, his mind supplied and rejected - before her disregard. “I am immortal!”
She laughed again, and Tom decided that he would learn to use that pitch and tone. It was more menacing that anything he’d heard. “I could shatter you, and that locket you wear, without straining myself. I have more power in a single breath than your precious Salazar used in his entire life.”
She paused, stepped a single, bare foot closer to him across the rocky hilltop, let her weight flow to that foot so she looked down into his eyes. “And none of it will help me. None of it. All of the power I have, all of the power that could kill you, could kill those you fear, is useless to me.”
“I fear no one!”
“You are a fool as well as a bad liar,” she murmured, breath ghosting across his face.
The mist was parting under the sun’s ponderous ascent, kicked away by the winds of her passing. This strange woman stood directly between Tom and the sun, her shadow beginning to bend its way across his face. His lips felt stiff as he whispered a harsh denial. “Death will not defeat me!”
“No,” she said, almost sad. “No. Death defeats no-one. Death is a lover, waiting to embrace you. Death is a long sleep after a life’s work, a fitting ending while your deeds in life still have meaning. Death will not defeat you, but in your foolishness, life might.”
She raised an arm to gesture to the hills and the rivers that emerged through the fading mist. “I have been here since these rivers followed a different course. I am Brigid De Tuatha De Danann. Your precious Slytherin, of whose blood you are so proud, was but a much diluted descendant of my family. To those without magic I was a Goddess.”
As she spoke she seemed to grow without gaining size, until the sun, the mist, the mountains faded from his consciousness. “And now,” she said, diminishing, “I’m a paragraph in a few books, barely mentioned in a classroom of dozing teenagers. That is true immortality, living when all purpose has faded beyond grasp, bound to a world that you cannot flee, watching the generations of your bloodline fade away into ignoble idiocy.”
She turned away from him then, facing out across the mist-filled valleys. “I will not tell you what you seek,” she said. “I will not allow another of my bloodline to disgrace himself, so. The path you seek to follow will lead not to immortality, but to a body that lives ever on, a soul splintered across the world, while your mind melts into senility and disrepair until somebody has the mercy to release you.
“It will be a fit ending for one who turned aside my advice.”
Tom stared at her, wondering when her sense had developed into this madness. She was too attached, he thought. Too attached to her offspring, and theirs. She allowed humanity to force itself upon her immortal power.
“I will release you, if you will but tell me how,” he said, making a show of humbleness.
She laughed coldly. “I am not naive, boy. You cannot release me. Even my own power is too slight for that. Return to your fake immortality, your false sense of superiority, Muggle child.”
The face she turned to him was that of the Goddess Brigid, and he fled down the narrow path, though he held himself to a walk. She is wrong. Wrong! She knows nothing…
Her laughter floated through the mist-clear air, echoing from stones no older than her.
She is nothing but a foolish, lonely old woman.