Devil of All Earth

"1,820 years ago, our ancestor, Ymir Fritz, made a contract with the Earth Devil to gain a power. The power of the Titans."

- Mr. Yeager tells his son of Ymir's tale

The Earth Devil (大地の悪魔) was 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" (悪魔の末裔) because of this belief.

The world was convinced that the Earth Devil 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.

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, whereas in the book Mr. 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.

In a bronze statue in Liberio depicting Helos slaying the Devil, he takes on a much less demonic appearance, becoming more Titan-like with a thick beard and a full head of hair, lacking visible horns.

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. Eren Kruger later claimed to Grisha Yeager that this myth was false, and that he believed Ymir had, in reality, discovered the source of all living matter, where she gained the power of the Titans. However, Kruger also claimed that there is "no such thing as truth in this world," indicating that it is possible for neither story to be true, as both contradict each other and tell a different tale.

At the time when Historia Reiss, a direct descendant of Ymir Fritz, was growing up on her family's farm in Wall Sheena, her half-sister Frieda Reiss visited her frequently, teaching her important life lessons such as reading and writing. Occasionally, Frieda would teach Historia the story of Ymir and the Devil, instructing her on how to be a loving and beloved girl as Ymir was.

Return to Shiganshina arc
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.

Marley arc
During the Tybur family's visit to Liberio, Willy Tybur visits a Marleyan estate to see the statue of Helos slaying the Devil.