Harry Potter in azkaban
The whole Weasley family and Hermione, Where killed by Death Eaters(including Snape).
The death eaters after killing everyone Used a complicated Ritual To Frame Harry Potter. (You can choose a different way to frame him)
Harry gets his wand snapped and is sentenced to life in Azkaban.
Harry is furious at Snape and the death eaters and swears to himself he will not die until he has found and killed every last one of them.
Harry is not suprised to find out that the magical community has believed what they were told. The only people who know he wouldn't do it Is Remus, Sirius, Neville, Luna, Flitwick, Mcgonagall.
For years Harry's friends and Loved ones have been secretly planning a way to break harry out of Azkaban.
The manage to find a way. (you choose how)
And then harry hunts down and kills Snape and every single death eater Using their deaths to power a very ancient and forbidden Ritual that allows Hus knowledge of the future to be sent back to his past self. Harry dies when his all his magic is ducked out to guide and power the ritual. (it uses Harry's magical signature to guide the knowledge to the right person And His magic helps power the ritual.)
Only the knowledge of what is going to happen travels back in time. So harry still has the mind of a child not a grown adult. But with the knowledge of what is going to happen unless he stops it, Harry starts to actually try in class. (harry has always held back Hus intelligence Beacuse if he was better than Dudley he would get beaten, locked in his cubourd and go without food)