"I Guess I Became the Mother of the Great Demon King's 10 Children" is a fantasy romance manga that centers on the life of a young woman named Aria, who unexpectedly finds herself in a strange and magical world. After a chance encounter with the Great Demon King, Zagan, Aria is thrust into a role she never anticipated: becoming the mother of his ten children. The story explores her journey as she navigates the challenges of raising a large family in a world filled with magic, demons, and political intrigue.

Aria, initially overwhelmed by her new responsibilities, gradually adapts to her role, forming bonds with each of the children, who possess unique personalities and magical abilities. The children, ranging from mischievous troublemakers to quiet and introspective individuals, add depth to the narrative as Aria learns to balance their needs while managing her growing feelings for Zagan. The Demon King, though initially aloof and enigmatic, reveals a softer side as he grows closer to Aria and their family.

The plot intertwines slice-of-life moments with larger conflicts, as the family faces external threats from rival demon factions and human kingdoms. Aria's human background becomes a point of tension, as her presence in the demon realm challenges long-standing prejudices and traditions. The story also delves into the complexities of Zagan's past, his responsibilities as the Demon King, and the sacrifices he must make to protect his family and realm.

Themes of love, family, and acceptance are central to the narrative, as Aria and Zagan work together to create a harmonious life for their children. The manga balances humor, emotional moments, and action, creating a dynamic and engaging story. The evolving relationships between the characters, particularly Aria and Zagan, drive the plot forward, offering a mix of romance and fantasy elements that appeal to fans of both genres.