Eldia was a nation that was located on Paradis Island which was chiefly populated by the Subjects of Ymir, a race that used to be able turn into Titans if injected with a serum.
In ancient times, the Eldians used Titans to build their empire, slaughtering countless peoples and taking their land. Eventually, the Eldian Empire grew unstable due to internal conflicts between the families with the power of the Titans, and collapsed during the Great Titan War a century ago. Eldia lost most of its land as well as seven of the Nine Titans to a newly reborn Marley, and many Eldians retreated and confined to its remaining territory of Paradis Island off from the coast of the mainland. King Karl Fritz raised the Walls to protect Eldia from Marley and the other nations of the world, and erased the memories of Eldia from the Subjects of Ymir inside.
After discovering the truth from Grisha Jaeger's basement in the year 850, the Eldians on Paradis Island embraced their true identities. Although they were officially the nation of "Eldia," the ultranationalist Jaegerists wanted to institute a New Eldian Empire by the time of the Rumbling.
History: In ancient times, the people of Eldia existed as a primitive tribe. The Eldian tribe pillaged and raided opposing peoples, even cutting out the tongues of those they took for their slaves and gouging out the eyes of disobedient slaves. Around 2,000 years ago, a slave girl named Ymir gained the power of the Titans.
Story: Upon finally reaching the basement in the Jaeger family household in Shiganshina, the Scout Regiment discovers three of Grisha Jaeger's books, describing all his knowledge of the history of Eldia and his own experiences in joining the Eldian resistance in Marley. These findings are later discussed in a meeting between the surviving members of the Scout Regiment as well as the highest military leaders, where the nation of the Walls is acknowledged by its people as Eldia for the first time in over a century. In 851, the Eldians of Paradis Island come into contact with a group of rebelling Marleyan soldiers, the Anti-Marleyan Volunteers.
Trivia: The emblem of Eldia is a nine-pointed star, most commonly found on the armbands of Eldian citizens in Marley. Nine is also the number of Titan powers created after Ymir Fritz's death. However, the ancient King Fritz wore a crown decorated with five nine-pointed stars about 2,000 years ago, even before the existence of the Nine Titans.
Source: Eldia (Anime), available under CC BY-SA 3.0.