Tartarus

Formally the Special Prison For Villain Criminals, Tartarus holds the country's most dangerous villains far from any civilian population, under the highest security standards available, and interrogates them there. The Police Force controls the site, run through a collaboration with the Hero Public Safety Commission and Pro Heroes.

The facility occupies an island roughly five kilometers from the mainland, ringed by enormous walls. One long bridge is the only way in, its entrance manned by guards who check every vehicle, and the gate on the mainland side is called The Bronze Gate; the anime shows the bridge can be collapsed one section at a time. Inside, blast doors line narrow steel-plated corridors.

Inmates are divided among six cell blocks according to how dangerous their Quirks are and how severe their cases, and the worse the individual, the deeper the cell, down to B10 at 500 meters below sea level. Each prisoner sits strapped into a large high-tech chair inside a full-body straitjacket. Sensors track vital functions constantly and respond to the faintest sign of unusual behavior or an attempt to fire a Quirk, feeding four ceiling-mounted automatic guns that open up on anyone who moves wrong or even seems to reach for their power. Guards watch the cells on screens, have their own conversations recorded on duty, carry weapons, and may use lethal force on anyone resisting recapture. Though called a prison, the place works as a dumping ground for anyone who has threatened public safety on a drastic scale, held regardless of sentencing status.

Visiting rooms put glass between prisoner and visitor while guards observe. Microphones carry the conversation, and computers read the prisoner's brainwaves so staff can gauge their state of mind as they talk. Visitors may not pass along news from outside and their time is capped, while heroes and police may interview inmates for investigations. Some prisoners are kept alive by machine, as All For One was with an oxygen mask, IV fluids, and feeding tubes; others are physically restrained or sedated, and reddish-orange jumpsuits mark them all. The facility is understood as the dark underside of Quirk society, and the saying holds that nobody sent there leaves alive. One guard scoffed at calling the inmates people, seeing beasts wearing human skin, and staff have feared that a single leaked conversation could expose them, given accusations of human rights violations.

Tartarus was built while chaos from the Meta Ability uprising gripped society and the vigilantes who became heroes needed somewhere ordinary jails could not match. Life sentences kept the most unstable away from any community, and hero defectors who turned on the Commission ended up there too, including its former assassin Lady Nagant. Stain arrived after the Hosu Incident, Muscular and Moonfish after the training camp assault, All For One after Kamino, Overhaul after the Shie Hassaikai Raid, and Kurogiri after Gran Torino and Naomasa Tsukauchi caught him. All For One rated the defenses so highly that he judged a fully completed Tomura with Gigantomachia beside him would still struggle to get through.

He got through anyway. Piloting Tomura's body after the Paranormal Liberation War, backed by Near High-End Nomu, he freed as many prisoners as he could to start a riot, letting the body he was piloting rejoin his original one. The pair then hijacked an emergency aircraft and hit other prison facilities, releasing more villains. The Nomu and the villains who swore allegiance to him wrecked the facility, and it was abandoned once they fled.

All For One is the prison's defining inmate, held in its lowest cell under maximum security despite his decrepit state, and visited there by All Might about a month after Kamino. Stain, Muscular, Moonfish, Overhaul, Kurogiri, Lady Nagant, Dictator, Ginji, Kunieda, and Gashly Eijiju also served time, and most were later recaptured, killed, or ended up assisting the heroes.

Among the staff, the guards Gyges and Briareos both died, as did Seiji Shishikura's father. Their names come from the Hecatoncheires of Greek myth, fitting a prison named for the pit beneath the Greek underworld, though the English dub reassigns the ruling god to Hades.