Himitsu no Mori no Majutsushi wa Nodoka wo Negau

Even while sealed, the mage Shagar struggles with the overwhelmingly powerful magic that overflows within him. To manage it, he has occasionally transferred his magic through sexes. One day, he meets a young runaway named Fio in the forest, who asks to become his apprentice. Shagar decides to take him in—but what will come of it...?

(Source: Takeshobo, translated)

Note: Includes 2 extra chapters.