Basically the Order of the Phoenix is losing and really needs a hero so they do the spell and what not but instead of Harry they get very powerful old witch a few years younger than Dumbledore named Ariana is who they get.

Except in this universe Ariana is a magical prodigy and never had a reason to become an obscures (did I spell that correctly) so she was never there to dies and break up the three way duel. The duel still happens because Aberforth is angry Albus is choosing Gellert over his own family.

Grindelwald is a little smarter and chooses not to use unforgivables (or has already manipulated Albus too much for him to care) so Aberforth loses the duel. Either way Albus chooses Gellert in the end and they were knows as “the Dark Lords Grindelwald and Dumbledore”.

So there you go. Maybe it’s Ariana who eventually takes down Albus and Gellert in then end.