Extreme Isolation

Extreme Isolation is an area of Arkham Asylum located within the Intensive Treatment Center. It is reserved for the most dangerous of inmates and is considered the most secure area on Arkham Island. It is notably one of the only parts of Arkham that Batman is unable to get into during the game.

Arkham Asylum Incident
At some point after creating the first Titan Henchman, Penelope Young banished the monster to Extreme Isolation out of fear. Later the henchman was released by Joker who had lured Batman into the room leading to Extreme Isolation's entrance.

After a battle in which Batman was thrown around the room, the henchman unexpectedly died of a heart attack. Joker, disappointed by this development offered to let Batman kill him. When Batman predictably refused Joker escaped into Extreme Isolation while Batman was unable to follow. After this it was never mentioned again.

However, in the DLC of Arkham Asylum and Batman: Return To Arkham there is a challenge map called Heart of Darkness that possibly offers a glimpse into Extreme Isolation.

Arkham Knight Incident
When the Joker Infection was beginning to take over Batman's mind, he turned the tables on his archnemesis and trapped him in one of the cages leading to Extreme Isolation. With one last goodbye to his enemy Batman finally defeats Joker and traps him forever.

Known Inmates

 * Titan Henchman (deceased)
 * The Joker (possibly)
 * Scarecrow (possibly)
 * Victor Zsasz (possibly)

Trivia

 * Extreme Isolation is the only area of Arkham Island never fully seen as Batman is unable to enter.
 * This also marks the first time Batman is unable to break into an area in the Arkhamverse.
 * While never confirmed, it was implied that Joker and Scarecrow were kept here.
 * This could have been one of the places in Arkham where Joker kept Jason Todd, especially given that it is inaccessible.