WARNING: The following contains spoilers for My Hero Academia Chapter #298, by Kohei Horikoshi, Caleb Cook and John Hunt, available now in English from Viz Media.
In the My Hero Academia universe, captured villains are typically sent to prison, but the vilest of criminals get sent to Tartarus, a special maximum-security prison separated from the rest of society. The name "Tartarus" derives from Greek mythology, an underworld level where evil people are imprisoned and punished after death.
Staying true to that origin, My Hero Academia's prison has been described as the most secure place on earth and has since evolved into a wasteland for the most twisted of individuals. In light of the manga's current prison break arc, let's go over what made breaking out of this prison such a big deal, to begin with.
In My Hero Academia, Tartarus is a large, heavily armored building located on its own island approximately five kilometers out at sea and extending up to 500 meters below sea level. A bridge connecting to the mainland is its only entrance. It harbors a wide range of criminals so wicked that the death penalty is deemed insufficient punishment. The more dangerous the individual, the deeper they are kept within the prison. They are restrained to high-tech chairs with straitjackets and monitored 24/7 with extensive countermeasures in place to keep them from using their Quirks.
In addition to copious restraints, the vital functions of each inmate are monitored. If they even think about activating their Quirks or make any sudden movements, machine guns mounted on the walls will fire without warning, which we learn when All Might visits All for One. Each prison guard carries lethal weapons and is authorized to use them in the event that a prisoner attempts to make a move.
While the prisoners can have visitors, all conversation is also observed and monitored by prison guards in specified visiting rooms. This is because their knowledge of the outside must be kept to a minimum. These visitation rooms separate the prisoner and the visitor through a large glass wall with all precautions from the cells still in place, ensuring that the inmate's mentality remains passive.
Tartarus' most notorious villain, All for One, holds such immense power that he was stuck underground in the prison's most secure cell block. While known to be Tomura Shigaraki's mentor, it is hinted that he has been working towards a much larger goal that has yet to be revealed. All for One was defeated and captured by All Might in the Hideout Raid arc and, until the latest chapter of the manga, has been in Tartarus ever since, restrained and monitored all the way down to his brainwaves. But he's not the only infamous resident.
The vigilante Hero Killer Stain (Chizome Akaguro), whose strong ideology of purging fake heroes for the sake of changing society has influenced many other villains, was been locked up and heavily observed in Tartarus since his defeat in the Hero Killer arc. Muscular was recruited by the League of Villains and assisted in infiltrating the heroes' training camp; his Quirk was likely subdued in Tartarus to keep him from escaping.
Overhaul, the leader of the Shie Hassaikai, lost both of his arms to Tomura and Mr. Compress, meaning he can no longer use his Quirk, but was still monitored as heavily as every other inmate. The cannibalistic villain Moonfish was defeated by Shoto and Katsuki Bakugo and placed on Tartarus' death row.
The inmate Kurogiri was groomed to be All for One's successor. He was captured by Gran Torino while meeting with Gigantomachia, a known accomplice of All for One's. In Tartarus, he was discovered to be a nomu and the corpse of Oboro Shirakumo, an old friend to Eraserhead and Present Mic. He is currently being restrained and interrogated to obtain more information on the League, nomus, and whether or not bringing Oboro back is a possibility.
As of Chapter #298, "Sounds of Collapse," of My Hero Academia, All for One has escaped Tartarus. Thanks to careful planning, an army of high-end nomus attacked from both inside and outside the prison. At the same time, he launched an attack that short-circuited all electronic devices causing a riot and freeing every prisoner in the process.
Tartarus has fallen at the worst possible time. Just as Hero Society is collapsing in on itself, all of the aforementioned criminals have regained their freedom. We'll just have to find out if they can be rounded back up and sent back to the hell they broke out of.
