NTR Kishi

Based on the erotic doujin game by Sensouya-san (戦争屋さん).

Coleus, a knight-in-training, attends a training academy. His new instructor turns out to be his childhood friend, Artel. "It's been a long time, Coleus." Coleus rejoices at reuniting with Artel, who has grown into a beautiful and strong woman. However, Artel quickly incurs the wrath of a fellow instructor, Lock. Lock, plotting to bring Artel down, seizes her wicked hands and robs her of her mental freedom. In front of her childhood friend, her voluptuous breasts are ravaged, and her virginity is taken. Lock's brutal acts escalate, and while Artel remains unconscious, her body, trained as a female knight, is transformed into that of a female. The joy of their reunion is the beginning of a nightmare. The proud knight's soul is corrupted (NTR) by a despicable trap.

(Source: Getchu, translated)