The remaining piece of Starro known as Jarro who's been helping the Justice League is giving a monologue about the universe and that there are some worlds that were created where the inhabitants are at constant war. Only one creature was ever known known to have survived that world. The creature's name was Starro.
It was released on the universe after evolving past what it was and took control of thousands of beings. Even going by the name Blackstar for a short time.
The present day has Jarro ,now dressed as Robin, spying on the Legion of Doom while the Justice League regroups after Lex Luthor absorbed Martian Manhunter. Hawkgirl still blames herself.
Jarro attacks the legion after they mock him and he claims that he's Batman's favorite Robin. Starro remembers all of the forces the Legion as in their possession.
Just as it seems he's against the wall the Justice League arrives and they're less than happy to see that the Legion is trying to hurt their reformed companion.
Jarro believes that the only way to make them stop fighting is to get them on common ground and transforms into his full Starro form.
Starro takes control of the Legion and League when Batman tells him that this isn't the way.
Batman explains that Jarro isn't villainous tyrant that he once was and that he's more than just Starro. There's so much more to him than his legacy and Batman tells Jarro that he's willing to help him realize that.
They hug with Jarro now understanding life and how he live his as a hero rather than give into the darkness that he was born from. We then go to the Legion of Doom headquarters where the Legion have a portion of the original Starro. They're regrowing him as Jarro will never grow to be as powerful as the original.