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.
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. As Eldia lost more and more territory to the Marleyans as Karl wanted, the King gathered as many people and members of the royal family as was possible before turning various of them into Colossus Titans to form three centric Walls around their last piece of undisputed territory in the year 743. At the same time Karl used the Founding Titan to erase the memories of his people so they would believe the entire human race except for them were devoured by the Titans.
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.
Paradis Island in the 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.
Source: Attack on Titan Wiki, available under CC BY-SA 3.0.