Fritz was the king of Eldia around 2,000 years ago. When one of his slaves, Ymir, gained the Power of the Titans, he used her to expand Eldia into an empire and destroy his enemies. As a "reward," he made her his concubine and had three daughters with her. He and Ymir are the ancestors of every Subject of Ymir.

Upon Ymir's death, Fritz forced his daughters to eat their mother's body to preserve her power. Upon his own death, he commanded his daughters to continue reproducing and eating each other, so that Ymir's people will never die out and Eldia could rule the world with its Titans forever.

(Source: Attack On Titan Wiki)