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In my story, I will have a hero begin a journey. It will be the underdog story as well as good-vs-evil story. Think Frodo vs Sauron for massive oversimplification. Except in my story the hero becomes corrupt. The hero wins. The hero becomes the villain of the next story. And this all happens fairly early. Approximately one-third of the way through the story.

Then the new hero has to overcome the old. They have a journey that parallels the villains journey only they retain their humanity. The second journey needs to take at least as long as the first and I want to develop that journey equally (if not more.)

Should the twist be left out? Make it clear from the beginning that the first hero is going to be the villain? Is it too soon in the tale for a reveal?

Have you ever played the game Diablo? In it, a knight comes to battle evil. A young prince has been possessed by a demon and much needs setting right in the realm.

At the end, when the player has finally defeated Diablo, there is a cut scene where we see the prince freed from the demon and our character is holding a stone. This stone is the essence of evil and is powerful. Our character succumbs to temptation and becomes the next evil to be fought in D2 & D3.

It was a shock, but worked.

I think you should gradually move your hero into darkness and let the deciding moment be something that, while unthinkable, is necessary for the defeat of evil.

Another example comes to mind. In 24, Jack Bauer went from an honest, by the rules CTU agent with a desperate problem to someone who understood the necessity of evil when you are fighting the good fight.

In Falling Down, the Micheal Douglas character was unaware that he had become a villain - he was just trying to get to see his daughter.

The timing might be a little early. It depends on your understanding of hero one - was his corruption slow and gradual or a quick slide down the slope? If it took time, give it time.

Hero 1 goes through the gauntlet to become a hero, and it's left him bitter. He's angry at how unfair it was. How many good men died just to prove purity (or whatever). He is a hero, but he's broken.

Hero 2 goes through the gauntlet to follow in his idol's footsteps: Hero 1. He gets through the gauntlet because he keeps reminding himself about Hero 1. He's got a symbol of hope.

However you tease the "twist" to the reader that this is not going to go as planned is up to you.

Hero 2 can hear bad things about his idol and deny it. Hero 2 has to put Hero 1 on a pedestal to preserve the symbol of hope, so he willingly ignores a few bad things that the reader knows is objectively true.

Meanwhile Hero 1 has been observing Hero 2's progress and has mixed emotions. He's probably going to die a meaningless death like all the others, or emerge jaded and broken just like himself. But as he comes closer to finishing, Hero 1 is emotionally invested, maybe contemplates cheating, intervening for Hero 2…, or is he suppose to stand back dispassionately? While Hero 1 is debating it (he's a broken hero), Hero 2 sees "a vision" of Hero 1 watching over him. Hero 1 is like "oh whatever, just die" and leaves, but Hero 2 is like "Hero 1 is my guardian angel! He watched over me and knew I could make it!"

By the time Hero 2 finishes the gauntlet, the reader knows these guys have very different ideologies, and those ideologies can't survive meeting each other. 2/3rds of the novel they finally meet and readers are excited, because they are in on the "twist", but the two characters aren't.

The last 3rd of the story is how this is reconciled. Hero 2 is disillusioned when his ideology falls, but it was flawed to begin with. He's got to find his own strength, not just idol worship. Meanwhile Hero 1 has either created a self-fulfilled prophecy that the gauntlet is a bad hero-maker, or he realizes that a hero is more than winning challenges it is a symbol of hope and he has a path to redemption.

You still get the twist but it's not about tricking the reader, it's about tricking the characters.

Probably opinion-based, but consider the goal is not to "trick the reader" but to put your MC in agony and conflict. Everything they worked for and believed in up to that point is stripped away! The reader is turning pages because they know that a big reveal is coming, and they can't wait to watch this collision. Let your reader in on the twist (by stages), don't tell the characters it's coming. You get suspense which is a better build, and there's still a twist because the characters will do SOMEthing – we have no idea what – but it's more about wrecking these characters' lives.

I don't think you should leave the twist out, though it might be a little too soon to incorporate twist. You also should not make it clear from the beginning that the hero is corrupt, instead showing it little by little.

No, not at all... It's the plot twists that keep the readers engaged to the story. Show him good in the start. Then, as the story unravels, show what makes him go corrupt, and describe his transition from the good to bad side slowly, chapter by chapter. And use the climax or the last fight to show that bad, corrupted side of him completely.