Around 300 years ago, Lailah, Emilia's mother, met and rescued a young Satan from the brink of death upon the destruction of his tribe by a random rogue demon. She oversaw his recovery back to full health, sharing with him much knowledge in the process. Before she left, she gave him a purple crescent-shaped crystal. During the course of the next hundreds of years, Satan rose as the Demon Lord, recruited other demons as his army, and gradually united the demon race into one single empire using the knowledge Lailah imparted to him. He planted the crystal in a garden within his castle, and it grew into a tree. All that time, Satan had no idea that it was a Yesod fragment.

(Source: Hataraku Maou-sama! Wiki)