The following contains spoilers for My Hero Academia Chapter 357, "Wounded Hero, Burning Bright And True" by Horikoshi Kohei, Caleb Cook and John Hunt, available now in English from Viz Media.
All For One's unparalleled intelligence and calculated approach to victory have helped him endure to continue his villainous campaign in My Hero Academia. However, it's his ability to steal the Quirks of others and incorporate them into his own that has always made him so dangerous. There are several Quirks that would spell a worst-case scenario for the heroes if All For One managed to steal them -- and Chapter 357 heavily hinted that he might've gotten his hands on one of the most dangerous. Following an incendiary defeat at Endeavor's hands, All For One resurrected himself with some variant of the Rewind Quirk.
Rewind is an incredibly powerful mutant Quirk wielded by Eri, the young former ward of yakuza leader, Overhaul. Her Quirk allows her to revert living things back to their previous states, but its actual workings are more complicated and mysterious. Once she learns full control, Eri might be capable of feats ranging from devolving humans back to their ape-like ancestors to simply healing recent wounds as if they never happened. Rewind was exploited to create the Quirk-erasing bullets Overhaul used against Mirio, and the effect could only be removed by Eri herself.
Given Rewind's almost limitless application, the apprehension many My Hero Academia fans felt upon learning it might be in All For One's hands is warranted -- but might be a little premature. His Quirk only allows him to steal those of whom he's had direct physical contact with. He also hasn't been shown exhibiting a horn similar to Eri's like Monoma did when he briefly Copied the Quirk. All For One definitely doesn't have Rewind itself but nevertheless, his sudden regenerative properties prove he has somehow stolen a sliver of Eri's power.
From how he spoke about the power-up he just received, it's unlikely his regeneration will be a permanent buff. All For One only activated it as his very last resort, and didn't seem to hold much stock in its efficacy. He mentioned that his body had already served its purpose, and with his Quirks in active rebellion against his wishes, trying out this new ability couldn't possibly make his situation any worse.
The million-dollar question right now is exactly how All For One is managing to heal. The only contact he could have had with Eri is through the few remaining permanent Quirk-erasing bullets Overhaul made with her cells. Evidently Shigaraki and Dr. Garaki's tests on them yielded some results, as the ability All For One is currently displaying couldn't be farther from the bullets' original purpose. All For One's healing also raised another point of concern: a singular eye and ear were the first body parts to fully materialize, but the villain lost those senses eons ago. It's possible that, for a brief moment, the Demon Lord in his prime condition might be resurrected on the battlefield.
All For One is My Hero Academia's king of being overly prepared. There's no telling what he could be planning following this shocking new ability reveal, and that's what makes him such a terrifying villain. Until he displays the upper limits of his new power, fans can only speculate how much of an impact he's going to have moving forward and hope Endeavor -- and the rest of the heroes -- are up to the task of defeating him.