WARNING: The following contains spoilers for My Hero Academia #297, by Kohei Horikoshi, Caleb Cook and John Hunt, available now in English from Viz Media.
In the My Hero Academia manga, the dust has only begun to settle on the battlefield of the war between the Paranormal Liberation Front and the united hero forces of Japan, but All For One is giving the heroes no time to rest as he immediately pivots to his next attack in Chapter #297. By possessing the body of his successor Tomura Shigaraki, he takes command of seven nomu and proceeds to assault the legendary prison known as Tartarus.
This prison is a maximum security facility designed for Japan's deadliest villains, with seven levels extending deep underground; the deeper a prisoner is, the more dangerous their powers. The possessed Shigaraki arrives at Tartarus with a single purpose: to free All For One's true body.
All For One launches an all-out assault on the prison using perfectly coordinated maneuvers between the two bodies under his control. Because Shigaraki is heavily injured and All For One's true body is hooked to a ventilator, however, he knows the smartest strategy isn't to handle everything himself. All For One instead frees all of Japan's most fearsome villains, destroying the heroes' fragile "victory" and delivering hero society its first real loss. But it's not just hero society that has lost here -- this is also a devastating blow to both Deku and All Might.
The prison break was perfectly timed to take advantage of the heroes' despair and disorganized forces in the aftermath of the War Arc. Turning what could be a considered a tie into an overall victory, All For One manages to free the villains imprisoned in Tartarus, a move that will ultimately let them wreak havoc on Japan. There are many familiar faces in their number, along with some new villains to the series. Moonfish, Muscular, Stain and Overhaul are among the familiar faces seen making their escape.
Alongside criminals Deku has already come face-to-face with, many of Japan's greatest villains have likely been freed as well. All Might and Endeavor have both spent decades locking up the most powerful villains only for All For One to now undo their efforts in one night. Hawks' father, a serial robber and murderer who Endeavor captured, may also escape if he's in Tartarus. It's clear that Japan may soon be rife with villains of a high enough caliber that only All Might and possibly Endeavor -- both likely out of commission now -- were previously able to handle them.
Chapter #297 of My Hero Academia also shows one new villain. As the prisoners all make their escape from Tartarus, a mysterious woman frees Overhaul out of pity in the chaos (lacking arms, Overhaul was unable to turn the wheel to open his cell door by himself). Nothing is known yet about her or her Quirk, but considering she received a significant amount of panels in the chapter and is locked up among known threats, it's likely she won't be a mystery for long.
Despite the hordes of villains that have been freed, the most calamitous blow for the heroes is All For One's return to the criminal underworld. All Might spent the last embers of One For All to capture him in Kamino Ward, and it now seems this sacrifice was for naught. All Might has fallen and All For One is now free to reign over the criminal underworld openly. Truly, dark times are ahead.