Eldia

, also known as the "subjects of Ymir", are a mysterious group from outside the walls.

History
Over 60 years before the fall of Shiganshina District, Ymir was cast out of her people, who "hated [her] just for existing,". She would later steal the power of Titan shifting from the Homeland, another society existing outside the walls. As a mindless Titan, Ymir would wander outside the walls for 60 years until the fall of Wall Maria, when the Homeland sends Reiner Braun, Bertolt Hoover, Annie Leonhart, and Marcel to breach the southern gate of Shiganshina District. After catching Marcel and consuming him, Ymir regained her humanity and became a part of the society within the walls.

Side Story: Ilse's Notebook
In the year 848, Ilse Langnar of the Survey Corps becomes the last surviving soldier of the left flank of the 2nd brigade during the 34th Expedition, lost in Titan territory within Wall Maria. With her vertical maneuver gear broken, she chooses to record her experiences as she makes her way north to Wall Rose. During her escape, she is caught by a Titan who corners her by a tree but refrains from eating her. The Titan speaks to her, first believing her to be "a subject of Ymir" before mistaking her for "Lady Ymir" herself. Ilse attempts to communicate with the Titan, asking it who it is and where it comes from, and why it eats humans. The Titan only response with pained moaning. When Ilse attempts to flee, the Titan catches and eats her as tears flow from its eyes. Her notebook was discovered a year later by Hange Zoë and Levi Ackerman.