Warning: the following article contains spoilers for My Hero Academia Chapter 333, "Specter," by Kohei Horikoshi, Caleb Cook and John Hunt, available in English from Viz Media.
Star And Stripe's high-stakes battle against Shigaraki and All For One has turned out to be one for the books to the very end. Star has demonstrated that she truly is the strongest woman around with a Quirk that's incredible beyond words. It's clear now why AFO said obtaining One For All was only his mid-game goal and what he meant when he claimed Star And Stripe would be both the greatest obstacle and ultimate opportunity.
All For One had his allies overseas stir up trouble in order to delay foreign heroes inevitably making their way to Japan. This was all so that he'd have time to perfect Shigaraki's body and steal One For All before challenging New Order. Once OFA belonged to him, all the other conspiring forces of the world, including New Order, would have been ants before him.
Star tries to use New Order to keep herself from decaying after Shigaraki touched her at the end of the Chapter 332, but it doesn't work -- the decay begins instantly and Star isn't able to prevent both it and the theft of New Order before it's stolen. So, in trying to protect her body, she ends up suffering both in the end.
After successfully stealing New Order, All For One attempts to assign a rule to the air but, before he can finish, a smiling Star smiling suddenly appears in his subconscious. What Star sees in return is a bizarre incomplete merging of Shigaraki and All For One's bodies. Their bodies are clearly conjoined, but both characters' faces and arms are still visible, showing that AFO is trying to fully fuse with Shigaraki's body while Shigaraki is trying to prevent that.
Back in reality, Shigaraki's body implodes as All For One's Quirks rupture inside of it. As it turns out, Star assigned a rule to New Order itself at the last minute, declaring that it will revolt against other Quirks. The result is essentially New Order raging against All For One's other Quirks inside Shigaraki's body. It's her last push to end this menace before she fully fades away.
Star sacrifices herself in hopes that All For One won't be able to keep New Order and her comrades will survive. All For One realizes that his Quirks are rebelling against each other and, since Quirks can't be destroyed, Shigaraki's body will continue to be ripped apart until it's completely annihilated. Desperate, he decides to force the hostile Quirk on one of the nearby pilots. However, the pilots don't give him the chance, as they seize the golden opportunity and all fire their lasers at him.
In her final moments before completely decaying, Star thinks of All Might. She acknowledges that she was able to live out her hero dreams because he saved her, and this is her way of paying him back. Chapter 333 concludes as Star rampages against All For One and Shigaraki in the subconscious realm, a true echo of her hero, All Might.