It starts with a SI into harry potter but dumped into orphanage. When he turns 11 he enters wiz world and finds out that Neville is bwl and his parents and sirius were killed by peter and bella trio.
He gets sorted into Ravenclaw and pretends to be an average student. He is loner except for Luna whom he treats like a sister, When Luna was killed during the begining of the seventh year(sixth&fifth year goes differently as neville has his gran for support and snape actually wants tom dead and is a competent spy).
Harry gets angry he leaves hogwarts and starts hunting down toms supporters. He is too brutal and kills anyone in his way so much that deatheaters fear him just like normal wizards fear tom. He even adds a Taboo to his name just like Tom so that he can hunt death eaters better. The DEs give him a title(You add it). He finally manages to take down most of the DEs while Neville with the support of albus kills Tom. Harry leaves britian to travel all over the world since he knows he will be persecuted in britian.
In an AU world (harry is dead but potters are alive) or It can be cannon world SI harry is summoned by the GOF during Tri-Wizard. He starts causing chaos and maybe live a good life there (FL can be Tonks). The novel directly starts from his summoning by the GOF and his SI life can be shown in parts as him telling it to other people . I did this because it creates a competent and powerful Harry with good logic too. You can also summon Neville with him so that his story can be revealed by him instead of flashbacks