So hear me out.
I've read quite a lot of Dark Harry Fanfics but most of them include joining Voldemort.
So please don't make Harry join Voldemort and instead create a 3rd faction (Dumbledore v Voldemorr v Harry)
You could also make Harry be from a different world (reborn) or sent back in time so he wouldn't allow himself to he controlled anymore. (This would be better because he would know who to trust and who to not truat so he will have an advantage)
Also make it go from 1st year at Hogwarts onwards, this will allow for more character development so it doesn't just happen instantly.
You could also have Harry absorb Voldemorts soul piece in his scar and have all of his memories including every single death eater, spell and secrets he may hold.
Don't make him the brother of the boy who lived because thats boring
Make Harry super cunning, manipulative and make him have a backbone.
Also, don't just make him like a copy of Voldemort, he doesn't have to kill anyone who disagrees with him. I just want a Harry that will use Dark Magics inorder to save the people he loves, to save himself froma life of misery.
Relationship Options: (My favourites)
1. Daphne Greengrass
2. Ginny Weasley (I just personally like her character and if you make Dumbles manipulative you could make her the only one who doesn't follow him) (I also think that due to Ginny's crush on Harry she would be trustworthy)