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I'm trying to identify a book and the series it was from, I read it on holiday and I believe it was a late entry in a long running series. All I really remember about the story was that the main character was male, and had some sort of epiphany while on a mountain near the end of the book. I think the character had some kind of spirit or other person in his head, and the epiphany was to do with accepting it or getting rid of it. The epiphany seemed to be a big deal and had an emotional impact for the mc. I feel like there were a lot of characters but being part way into a long series made it hard to get everything that was going on.

The series was definitely a fantasy setting with the main character being able to use some kind of magic. There was a Big Bad who was trapped somewhere and had chosen champions that were trying to free him.

Thanks to @Eshier for the guidance to the actual quote

This sounds like the Wheel of Time Saga by Robert Jordan. This epic fantasy series has spanned 14 volumes and over 20 years of writing/publishing.

The main character is Rand al'Thor, who starts off as a normal adolescent but as the series progresses discovers that he can perform magic, through wielding the One Power.

He hears voices in his head which are the manifestation of a man from a previous age of the world (called Lews Therin Telamon - the Dragon) and Rand becomes known as the Dragon Reborn.

The antagonist is the enemy of the Creator, known as the Dark One who was imprisoned at the beginning of the world, but was partially released. The Dark One recruited 13 champions, known as the Forsaken.

The section remembered by the OP seems to be Chapter 50 of the 12th book of the series, and this is where Rand Travels to the top of Dragonmount and has his epiphany with Lews Therin

Wind blew around Rand as he sat at the top of the world. His weaving of Air and Fire had melted away the snow around him, exposing a jagged gray-black tip of rock about three paces wide. The peak was like a broken fingernail jutting into the sky, and Rand sat atop it. As far as he could tell, it was the very tip of Dragonmount. Perhaps the highest point in the world.

He sat upon his small outcropping, the access key sitting on the rock in front of him. The air was thin here, and he’d had trouble breathing until he’d found a way to weave Air so that it compressed slightly around him. Like the weave that warmed him, he wasn’t certain how he’d done it. He vaguely remembered Asmodean trying to teach him a similar weave, and Rand hadn’t been able to get it right. Now it came naturally. Lews Therin’s influence, or his own growing familiarity with the One Power?

Dragonmount’s broken, open mouth lay several hundred feet beneath him, to the left. The scents of ash and sulphur were pungent, even at this distance. The maw was black with ash and red from molten rock and blazing fires.

He still held to the Source. He didn’t dare let go. This last time he’d seized it had been the worst he could remember, and he feared that the sickness would overpower him if he tried again.

He had been here for hours. And yet he did not feel tired. He stared at the ter’angreal. Thinking.

What was he? What was the Dragon Reborn? A symbol? A sacrifice? A sword, meant to destroy? A sheltering hand, meant to protect?

A puppet, playing a part over and over again?

He was angry. Angry at the world, angry at the Pattern, angry at the Creator for leaving humans to fight against the Dark One with no direction. What right did any of them have to demand Rand’s life of him?

Well, Rand had offered that life to them. It had taken him a great while to accept his death, but he had made his peace. Wasn’t that enough? Did he have to be in pain until the end?

He had thought that if he made himself hard enough, it would take away the pain. If he couldn’t feel, then he couldn’t hurt.

The wounds in his side pulsed in agony. For a time, he’d been able to forget them. But the deaths he had caused rubbed his soul raw. That list starting with Moiraine. Everything had begun to go wrong at her death. Before that, he’d still had hope.

Before that, he’d never been put in a box.

He understood what would be required of him, and he’d changed in the ways he thought he needed. Those changes were to keep him from being overwhelmed. Die to protect people he didn’t know? Chosen to save mankind? Chosen to force the kingdoms of the world to unite behind him, destroying those who refused to listen? Chosen to cause the deaths of thousands who fought in his name, to hold those souls upon his shoulders, a weight that must be borne? What man could do these things and remain sane? The only way he had seen had been to cut off his emotions, to make himself cuendillar.

But he had failed. He hadn’t been able to stamp his feelings out. The voice inside had been so small, but it had pricked at him, like a needle making the smallest of holes in his heart. Even the smallest of holes would let the blood leak free.

Those holes would bleed him dry.

The quiet voice was gone now. It had vanished when he’d thrown Tam to the floor and nearly killed him. Without that voice, did Rand dare continue? If it was the last remnant of the old Rand—the Rand who had believed that he knew what was right and what was wrong—then what did its silence mean?

Rand picked up the access key and stood up, boots scraping stone. It was midday, though the sun still lay hidden behind the clouds. Below, he could see hills and forests, lakes and villages.

“And what if I don’t want the Pattern to continue?” he bellowed. He stepped forward, right to the edge of the rock, clutching the access key to his chest.

“We live the same lives!” he yelled at them. “Over and over and over. We make the same mistakes. Kingdoms do the same stupid things. Rulers fail their people time and time again. Men continue to hurt and hate and die and kill!”

Winds buffeted him, whipping at his brown cloak and his fine Tairen trousers. But his words carried, echoing across the broken rocks of Dragonmount. It was cold and crisp, the air new. His weave kept him warm enough to survive, but it did not stop the chill. He hadn’t wanted it to.

“What if I think it’s all meaningless?” he demanded with the loud voice of a king. “What if I don’t want it to keep turning? We live our lives by the blood of others! And those others become forgotten. What good is it if everything we know will fade? Great deeds or great tragedies, neither means anything! They will become legends, then those legends will be forgotten, then it will all start over again!”

The access key began to glow in his hands. The clouds above seemed to grow darker.

Rand’s anger beat in rhythm with his heart, demanding to be set free.

“What if he is right?” Rand bellowed. “What if it’s better for this all to end? What if the Light was a lie all along, and this is all just a punishment? We live again and again, growing feeble, dying, trapped forever. We are to be tortured for all time!”

Power flooded into Rand like surging waves filling a new ocean. He came to life, glorying in saidin, not caring that the display must be brilliantly visible to men everywhere who could channel. He felt himself alight with the Power, like a sun to the world below.

“NONE OF THIS MATTERS!”

He closed his eyes, drawing in more and more power, feeling as he had only twice before. Once when he had cleansed saidin. Once when he had created this mountain.

Then he drew in more.

He knew that much power would destroy him. He had stopped caring. Fury that had been building in him for two years finally boiled free, unleashed at long last. He spread his arms out wide, access key in his hand. Lews Therin had been right to kill himself and create Dragonmount. Only he hadn’t gone far enough.

Rand could remember that day. The smoke, the rumbling, the sharp pains of a Healing bringing him back to lucidity as he lay in a broken palace. But those pains had paled compared with the agony of realization. Agony from seeing the beautiful walls scarred and broken. From seeing the piles of familiar corpses, tossed to the floor like discarded rags.

From seeing Ilyena a short distance away, her golden hair spread out on the ground around her.

He could feel the palace around him shaking from the earth’s own sobs. Or was that Dragonmount, throbbing from the immense power he had drawn into himself?

He could smell the air thick with blood and soot and death and pain. Or was that just the scent of a dying world, spread before him?

The winds began to whip at him, spinning, enormous clouds above twisting upon themselves, like ancient leviathans passing in the profound black deep.

Lews Therin had made a mistake. He had died, but had left the world alive, wounded, limping forward. He’d let the Wheel of Time keep turning, rotating, rotting and bringing him back around again. He could not escape it. Not without ending everything.

“Why?” Rand whispered to the twisting winds around him. The Power coming to him through the access key was greater than he’d held when cleansing saidin. Perhaps greater than any man had ever held. Great enough to unravel the Pattern itself and bring final peace.

“Why do we have to do this again?” he whispered. “I have already failed. She is dead by my hand. Why must you make me live it again?”

Lightning cracked above, thunder buffeting him. Rand closed his eyes, perched above a drop that plummeted thousands of feet downward, in the middle of a tempest of icy wind. Through his eyelids, he could sense the blazing light of the access key. The Power he held inside dwarfed that light. He was the sun. He was fire. He was life and death.

Why? Why must they do this over and over? The world could give him no answers.

Rand raised his arms high, a conduit of power and energy. An incarnation of death and destruction. He would end it. End it all and let men rest, finally, from their suffering.

Stop them from having to live over and over again. Why? Why had the Creator done this to them? Why?

Why do we live again? Lews Therin asked, suddenly. His voice was crisp and distinct.

Yes, Rand said, pleading. Tell me. Why?

Maybe . . . Lews Therin said, shockingly lucid, not a hint of madness to him. He spoke softly, reverently. Why? Could it be . . . Maybe it’s so that we can have a second chance.

Rand froze. The winds blew against him, but he could not be moved by them. The Power hesitated inside him, like the headsman’s axe, held quivering above the criminal’s neck. You may not have a choice about which duties are given you, Tam’s voice, just a memory, said in his mind. But you can choose why you fulfill them.

Why, Rand? Why do you go to battle? What is the point?

Why?

All was still. Even with the tempest, the winds, the crashes of thunder. All was still.

Why? Rand thought with wonder. Because each time we live, we get to love again.

That was the answer. It all swept over him, lives lived, mistakes made, love changing everything. He saw the entire world in his mind’s eye, lit by the glow in his hand. He remembered lives, hundreds of them, thousands of them, stretching to infinity. He remembered love, and peace, and joy, and hope.

Within that moment, suddenly something amazing occurred to him. If I live again, then she might as well!

That’s why he fought. That’s why he lived again, and that was the answer to Tam’s question. I fight because last time, I failed. I fight because I want to fix what I did wrong.

I want to do it right this time.

The Power within him reached a crescendo, and he turned it upon itself, drove it through the access key. The ter’angreal was connected to a much greater force, a massive sa’angreal to the south, built to stop the Dark One. Too powerful, some had said. Too powerful ever to use. Too frightening.

Rand used its own power upon it, crushing the distant globe, shattering it as if in the grip of a giant’s hands.

The Choedan Kal exploded.

The Power winked out.

The tempest ended.

And Rand opened his eyes for the first time in a very long while. He knew—somehow—that he would never again hear Lews Therin’s voice in his head. For they were not two men, and never had been.

He regarded the world beneath him. The clouds above had finally broken, if only just above him. The gloom dispersed, allowing him to see the sun hanging just above.

Rand looked up at it. Then he smiled. Finally, he let out a deep-throated laugh, true and pure.

It had been far too long.

The Gathering Storm - Chapter 50

Sounds like Brandon Sanderson's Stormlight Archives #2 Words of Radiance, especially when you mention at least 600 pages