Paradis Island was a massive island on which the three Walls were located, and Eldia's last territory. It was just off the coast of Marley and is surrounded by the ocean. Marley used the island in the past as a dumping ground for its Eldian criminals, turning them into Pure Titans as punishment. In this context, the island was often called "Heaven".
By the year 851, the island had been cleared of Pure Titans thanks to the use of the Executioner from Hell devices, allowing its inhabitants to roam freely outside the Walls.
Following Eren Yeager's initiation of the Rumbling, civilization advances on the island for many years before it is eventually destroyed by war.
History: Late into the history of the Eldian Empire, Karl Fritz, the 145th monarch of the Fritz family, chose to abandon the conflict over the Nine Titans between the subservient Eldian families on the continental mainland. Karl, ashamed of Eldia and its history of persecuting Marley conspired with the Tybur family to ensure Marley would win the conflict and overthrow the Empire while he moved the capital of Eldia to Paradis, a remote island at the time. In the King's absence, the Great Titan War continued to cripple the Eldian Empire as the eight Titan families would engage in an endless cycle of conspiracy and betrayal.
Story: Marley sends members of the Warrior Unit Reiner Braun, Bertolt Hoover, Annie Leonhart, Marcel Galliard to retake the Founding Titan. Marcel is eaten by a Pure Titan while the other 3 make their way to Wall Maria. Annie as the Female Titan gathers Pure titans while , Bertolt as the Colossus Titan breaches the outer gate of Shiganshina District, allowing countless Titans to enter the city.
Trivia: " If mirrored vertically, the continent controlled by Marley and the island of Paradis are very similar to the real-life continent of and the island of , respectively. The detailed map of Paradis Island shown in Chapter 98 reveals three rivers on the north end of the island moving inland from the western shores. These rivers roughly correspond with the -, -Mananantanana, and - rivers in Madagascar.
Source: Paradis Island, available under CC BY-SA 3.0.