The War was over, Harry had Defeated Voldemort, although it was hard to say they won. Voldemort's final play was to unleash the Hordes of hell into the earth with the final beat of his heart.
In the 10 years since then Harry once more lead the light against the inhuman monsters of the Hells, even with alliances with some of the more Intelligent members of Hell intent on stemming the flow and regain control the war was near impossible. That is until Fleur Weasley, Widow and raid captain found a stash of magical books and items that once were held by Voldemort himself.
Time Travel
But the magical need was too strong for one man, they would need many... They would need a Sorcerer and that Sorcerer would need a Coven around him. There was only 1 sorcerer Fleur would trust with this. Harry James Potter.
Harry and the Coven head back to start of the Voldemort war intent on ending the war before it can start as well as putting their Few Demonic Allies into a better position to prevent a future spill
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Harry/Fleur/Harem, Include some lesser seen characters like Su Li, The Carrow Twins and the like
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Harry and the Light aligned with the Succubi/Incubi from hell, who are tired of the war between the Wrathful and Prideful (That they are caught between and often die) which has spilled into the human world
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The Lustful send a Succubus to act as a Consort for Harry who is in the Harem and goes back in time with them (although they might have to summon her specifically again)
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Covens are very sexual and intimate, while they don't have to be permanent, due to the Covens history even when returning to a safe time period have become overly attached