In the series finale of "Akuyaku Reijou Level 99," the Kingdom of Valschein and its allies launch a decisive offensive against the Demon Lord, who has emerged and threatens the realm with an army of monsters. Yumiella joins forces with nobles, the royal army, and fellow students of the royal academy, including Alicia, Edwin, William, and Oswald, under the leadership of Captain Adolf and Prince Edwin, to confront the Demon Lord in his castle.

Before the battle, Yumiella receives a protective amulet from Patrick, highlighting its significance for the events to come. The alliance's approach to the Demon Lord's castle is fraught with challenges, including a direct confrontation with the Demon Lord himself, who flaunts his power, prompting Yumiella and the group to retaliate. However, the group's initial efforts prove insufficient against the Demon Lord's might. Yumiella demonstrates her strategic prowess and counters the Demon Lord's moves. Recognizing Yumiella's unique strength, the Demon Lord offers her a place by his side.

The Demon Lord recounts the story of his betrayal and transformation into a Demon Lord after being cast out by the first king of the kingdom. Unfazed by this revelation, Yumiella continues to fight for her friends and the kingdom.

The battle reaches its climax as Yumiella and Alicia join forces to oppose the Demon Lord. Together, they ultimately succeed in defeating him.

However, the victory is marred by Alicia's sudden attack on Yumiella, triggered by the game's storyline, forcing her to turn against Yumiella. Quick thinking and the protective amulet save Yumiella from a fatal blow. By attempting to conclude the game's plot with a happy ending—or through Edwin's confession of love to Alicia—Yumiella skips the part where she would have been a hidden boss opposing the heroes.

After the battle, the group returns to their normal lives. Yumiella reflects on the unexpected twists of her adventure and looks forward to her future endeavors. The episode ends with Yumiella and Patrick embarking on a new adventure, underscoring Yumiella's enduring desire for strength and discovery, even at level 99.