So I've got this character who is unknowingly the vessel for an evil spirit that belonged to one of the greatest threats to the world that my story has ever known; a sorcerer who sought to forge a perfect world order when he realized the warring factions of the world would eventually destroy the world. Basically a Darth Vader type who's aspirations were once noble, but quickly became corrupted due to their ruthless approaches to achieving their task (another good example for his progression from hero to villain is the portrayal of Superman in Injustice: Gods Among Us).
However, the sorcerer is eventually defeated by; himself. See, in the time he spent amassing his power, the sorcerer's original self (let's call him Bob) was still present, buried in his subconscious. As he began to become more of a villain as time wore on, everything that Bob used to be was subconsciously buried as a new personality emerged; the evil Bob (let's call him Steve).
So Bob and Steve are two sides of the same coin; they're the same person, but at different points in their lives. Bob is the kind-hearted, well-intentioned hero who set out to change the world for the better, while Steve took Bob's philosophy to an extreme and began to act ruthlessly to achieve their dream of a world without the constant fighting (but ultimately siding with the 'mostly evil' faction's ideals of control). Occasionally, Steve would have lapses in judgement where Bob would return as the primary consciousness, but Steve actively tried to force Bob down so that he could control their body. While Steve's busy conquering the world, Bob's trying to find a way to kill Steve.
Eventually, the heroes of the time (unknowingly aided by Bob, who manifests via astral projection) find a way to separate the two from each other (as there's no physical way to kill Steve without also killing Bob at the same time; and Bob's a cool guy). Either they'd separate him with a weapon, artifact, or some form of ultra-powerful group magic, but nonetheless; it happens. Bob is back, Steve is destroyed, and the world goes on.
Jump a few thousand years ahead, and the organization who followed Steve decides it's time to bring him back. As it turns out, his spirit survived the ancient battle, but he's just been floating around trapped in a demon realm and needs a vessel to return. Thing is; he can only be reincarnated through his natural descendant (just so happens to travelling with the MC).
The villains abduct the character (who has also been unknowingly helped in their relatively unrelated quest by great grandpa Bob's long-dead spirit) and Steve is brought back to the realm of the living. At this point, I could go one of two ways; either Steve now inhabits the body of the character and their consciousness is pushed aside like Bob was before, or Steve takes on a body of his own through some sort of ritual like how Voldemort did in Goblet of Fire. He then becomes another threat the heroes must overcome in their ultimate quest, which they didn't realize had been leading up to this point (basically everything they've encountered before has been manipulated by an even more powerful entity that Steve once worshiped, and Steve's resurrection is just a small part of a much grander plan).
The character of Bob would be with the group from the near-start of the story, as would references to Steve; but the reveal of their lineage wouldn't be revealed until the plot gets closer to Steve's reincarnation.
Tldr; An ancient villain is split into two halves; one good, and one evil. Evil one is imprisoned in a shadow realm, while the Good one lives out their days and dies of natural causes. Centuries later, Evil half wants to be reborn, but this can only be done through their descendant, which eventually works.
What are your thoughts on this concept? Do you think it could be an interesting plot development, or does it sound like it could easily become a competitor for the 'evil reincarnated edgelord of the week award'? Any criticisms, suggestions, or advice would be really appreciated!