Yagate Kuromaku e to Itaru Saiteki kaiやがて黒幕へと至る最適解; The Optimal Solution to Eventually Becoming the Mastermind

After losing her, and have finally returned to his previous world after completing his preparations, Kaltz vows to take revenge on those who have framed his beloved lifelong master, Altesia. And now, he has turned 10 years old. Using the knowledge he has gained in the future, Kaltz begins to operate in the dark as the "True Mastermind."