The Devil of All Earth is a figure of Eldian mythology who made a deal with Ymir Fritz, granting her the power of the Titans. The Subjects of Ymir are called the "children of the Devil" because of this belief.
The world was convinced that the Devil of All Earth was real, slain by the Marleyan hero Helos during the Great Titan War, until Willy Tybur revealed that the story of Helos is mere propaganda. Eren Kruger suspected that Ymir's deal with the Devil was fabricated by the enemies of her people, although the tale is told in an Eldian book as well.
In all illustrations depicting the Devil, it is shown as a large, crouching figure with a menacing smile. In the book Frieda Reiss reads to Historia, the Devil is covered in a large cloak and hood and has lipless mouth filled with sharp teeth, whereas in the book Yeager reads to Grisha, its head is exposed, revealing two goat-like horns and a long tail. It also appears with sharp claws, large muscles, and pointed ears.
Appearance: In all illustrations depicting the Devil, it is shown as a large, crouching figure with a menacing smile. In the book Frieda Reiss reads to Historia, the Devil is covered in a large cloak and hood and has lipless mouth filled with sharp teeth, whereas in the book Yeager reads to Grisha, its head is exposed, revealing two goat-like horns and a long tail.
History: Over 1,800 years before the Fall of Wall Maria, a girl by the name of Ymir Fritz is said to have met with the Devil, and the two made a deal. It is unknown what Ymir offered the Devil, but in return it granted her immense power, turning her into the first of all Titans.
How Devil of All Earth echoes later: Some years later, when the Survey Corps discovers Grisha Yeager's basement, they find three journals. Within Grisha's writings, he recounts his childhood when he was told the story of Ymir and her deal with the Devil as well as Kruger's revelation of the falsehood behind the myth. During the Tybur family's visit to Liberio, Willy Tybur visits a Marleyan estate to see the statue of Helos slaying the Devil.
Trivia: While Eldian mythology depicts Ymir Fritz as a woman who made a deal with the Devil, the ironic reality is that Ymir, a slave, entered into an unwanted arrangement with King Fritz to use her Titan powers to further Eldia's dominance and prosperity. In exchange, Ymir was forced to become the King's concubine as an unfortunate "reward".
Source: Attack on Titan Wiki, available under CC BY-SA 3.0.