Feast of Souls, by C.S Friendman.

This book is set in a world of medieval intrigue, where dread Magisters,  immortal male sorcerers of unholy power, serve the Kings of man to while away the long centuries, and plot dark machinations. Kamala is a young, hardbitten woman, full of promising magical ability, and has sought out a retired Magister, Ethanus, in order to attempt the impossible: become a female Magister. But the price to become a Magister is that you burn yourself out magically, unto death, and then reach beyond your own mortality.

So out in the nighttime forest, beneath a starry sky, Kamala attempts it.

…

(pg. 23)

He hears her scream. Not a sound voiced by her flesh, but an agonized howling of her innermost soul. It is at once defiance, fear, determination – raw stubbornness, which has always been her strongest trait. Yet even that is not enough now. You must be willing to leave behind what you are, he thinks, and become something so dark and terrible that men would cringe in horror if they knew it walked among them. And you must choose that course of your own accord, without being shown the way; you must want it so much that everything else is cast aside.

Does a man truly cast everything aside? he wonders. A woman must. Nature has prepared her to bring life into the world and nurture it, and the very essence of her soul is shaped to that purpose. Such a soul cannot manage Transition in its natural state, nor survive the trial of the spirit that will follow. Can Kamala strip herself of all that the gods gave her in making her a woman, can she hunger for life so desperately that the lives of others are as nothing to her? It is a trick men are born to, for Nature has fashioned them for war, but women must learn it unnaturally.

You were meant to bring life into the world, he thinks. Now, to survive, you must bring death.

She is on her knees now, shaking violently as spasms of dying engulf her soul. Ethanus can hear her desperation screaming out across the heavens. He even hears his name, voiced as a prayer – a plea for the information she needs to survive – but he makes no answer. Each student must find his own way to the Truth; that is the Magister’s tradition. To do otherwise may bring weaker students through Transition safely, but it cannot make them fit for what comes after.

Forgive me, my fierce little whore. And forgive the gods, who have decreed that all birth must be agony.

And then-

He can sense it in her. A sudden awareness of something outside herself. Beyond the clouds, beyond the wind, beyond the parts of the earth that man has given names to. A source of power outside herself, like but unlike the athra whose flow trickles to a stop within her soul. She grasps at it but it eludes her. No! she screams. I will not fail! Another spark takes its place and she focuses her will upon it, desperate to lay claim to it before her flesh expires. Ethanus can taste her determination on his tongue, the sudden elation of understanding. This, this is what she was mant to discover – this foreign spark that is not soulfire, but might be bound and made to take its place. Why did Ethanus not simply tell her that? Why has he not taught her the tricks she needs to tame it? Now she must wrestle with Death even as she races to weave a link between herself and this distant power, so strong that no force wielded by man or god can ever sever it.

And he knows it before she does, when she has won. He knows because he has watched other apprentices expire at this point, consumed at the very threshold of immortality. In them the final sparks within their own souls had died before they could claim this new power, and Death had dragged them screaming into oblivion. In her… the ice within her veins cracks… the strangled heart dares a new beat… the breath that has been all but choked off by the force of her trials draws inward once again, bringing warmth to her lungs. He knows what she does because he knows what signs to watch for. She… she knows only that awareness of a foreign power throbs within her now like a second heartbeat, and that her flesh draws strength from it, easier with each passing breath.

When she is sure of what she has done, and sure it cannot be undone, she looks at him. There are tears in her eyes, red tears, for her body has squeezed forth blood in it’s exertions. How appropriate, he thinks. There were tears in his own but he wiped them away before she could notice. He does not want her thinking to question what emotions spawned them.

“I live,” she says, and in that phrase are captured a thousand things unsaid. A thousand questions.

“Yes,” he responds. Answering them all.

…