Well here is my idea and you can add your input. I changed a little of the story around from the movies and books in accordance to Voldemort's and Bellatrix's demise. In the book of course Molly Weasley killed Bellatrix and Voldemort is incapable of love. Well I was thinking of Bellatrix having two children with Voldemort. Yeah I know lame but really the only enemy I could see as being a worthy enemy would be a relative of Voldemort. Anyway the two children are brother and sister with the brother fully holding all of Voldemort's and Bellatrix's views on pure-blood and things of that nature while the sister starts out believing it, then becoming disillusioned, then liking one of the Potter brothers, then turning against her own brother. Anyway I would play the brother because I like evil characters lol.
Oh by the way here is a little info about the two characters. My character (the brother) knows that Voldemort was a half-blood but he lied to his sister and told her that he was pure-blood thus trying to insure that she would be as fanatical as him about "keeping the bloodline pure." Originally they were given to an orphanage after Voldemort and Bellatrix died and the sister got loving Muggle parents but the brother gets abusive, alcoholic ones thus insuring that he would have a hatred of Muggles for what his foster parents did (like Voldemort hating Muggles because his Muggle father abandoned him) and insuring that he hates any pure-blood wizards who would "degrade themselves" by mixing with Mudbloods (pure-blood traitors) and having offspring (half-bloods). Basically as I said his view is the same as Voldemort's and Bellatrix's views on bloodline and the such. Anyway when he turned 16 he ran away from his abusive foster parents and convinced the sister to join him after showing her magic and that they were related thus the sister's foster parents search for days and never find her and presume her to be dead. Thus the story starts when the sister's 14 and the brother's 16 and they get to go to their first year at Hogwarts (no one knew of their existence until then).
Brother- Me
Sister- Kopalover
(I'll come up with names for them before we start).
Albus Potter-
James Potter-
Harry Potter-
Lily Potter-
Ginny Weasley-
Hermoine Granger
Ronald Weasley-
Rose Weasley-
Neville Longbottom-
Hadgrid-
Voldemort and Bellatrix (in flashbacks and hallucinations): Bellatrix- Shadowfax
(Any other characters from the book along with OCs are also welcome)
Mia Black- Shadowfax
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Salazar Riddle walked towards the train with his sister and started muttering, "Look at all of these filthy Mudbloods and half-bloods here! They make me sick ruining my perfect world!" He said, "Must be good finally going to school at your fourth year sister. I wasn't found out about until my second-to-last year." He looked like a 16-year old Tom Riddle in the flesh as he said to his sister who looked like a 14 year-old Bellatrix, "Just remember we pure-bloods are not only the smart ones here but since we have not poisoned our blood we are the most superior. That is why if you EVER have any relationship with a Mudblood or half breed I'll personally kill them and you myself. Ah besides there's nothing good about Muggles. Your Muggle parents abandoned you and my Muggle parents were drunkards and fiends who beat the tar out of me every day. No we should be proud of our bloodline sister but I am not completely heartless. I don't mind Muggles as long as they don't pollute our world hahaha. There should be a strict segregation of the species." As he finally stopped muttering he carried his and his sister's bags as he revealed, "I've been talking to our parents. I can talk to them. They told me to get as close as we can to the Weaselbums and Potters and finally get our revenge for our parents by killing their children. We'll not rest until this is accomplished. Only after we kill their children and they are in agony can we kill their parents nice and slow hahaha."
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Harry finally saw that his children were off as he looked at Albus and James go onto the Hogwarts Express and Ron and Hermoine saw Rose go off on the train. Harry then revealed to his friends, "I have a bad feeling about this year for some reason. I feel like...our children...could be in danger just by going to Hogwarts." Ron asked, "What do you mean?" Harry looked at his brother-in-law, sister-in-law, and wife Ginny and said, "Ah nothing. It's probably just me being paranoid. I just can't believe after all of these years Voldemort is finally gone." He paused, "Bellatrix and Voldemort and all of the other evil wizards are finally gone yet I still can't help but feel in the back of my mind they're still alive."
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Meanwhile Albus and James Potter and Rose Weasley finally got into a compartment as James looked at Rose's books and said, "Hey Rose are you actually reading all of those? I mean I heard of light reading but that's NOT it." James smiled as he looked at Albus, "Nervous already you dork? Geez we haven't even left the station yet and you're all nervous." Albus flushed and said, "I am not! I'm just wondering what will happen this year?" James said, "Ah nothing exciting I'm sure but hey maybe there are some girls to look at." Albus blushed, "Uh girls?" James rolled his eyes, "Ugh you're a bigger dweeb than I thought brother..."