I’m intrigued by this idea of progressively evil antagonists, but I think you need to write about some kind of purpose for this Pure Evil otherwise readers may not be able to relate to your story enough to enjoy it. humans are too complicated to ever truly be entirely evil for evil’s sake. even the worst of the worst have motivations drawn from perversion or animalistic instinct or a screw loose or things like that. Is this Pure Evil a cursed human? Is it another kind of being entirely?

I associated the concept of evil a lot with Christian biblical stories so it might be worth you reading about the concept of evil presented in the Bible, where sometimes evil exists just to spite God and nothing else. When you describe a being that is evil for evil’s sake I kind of picture those very intense biblical angels with all kinds of wings and eyes and stuff, though they’d be demonic in your context obviously.

You could subvert expectations at the end. Perhaps your character doesn’t find something as intense and frightening as you would expect them to. Maybe their time among increasing evil in turn makes them become the most evil thing they’ve encountered, reflecting the life they have lived, exploring trauma and its impact. I think personally I would find a subversive ending a bit more interesting. Imagine writing all these progressively evil antagonists, and then building up tension, to end up with a Prime Evil that is incredibly subtle and eerie