Despite Yuji's attempts to reach Junpei's heart and invite him to Jujutsu High, Mahito intervenes. Mahito uses his Idle Transfiguration technique to mutate Junpei’s body into a horrific form to force a fight with Yuji.
Yuji rushes to Satozakura High School to stop Junpei, who has been manipulated by the cursed spirit Mahito into attacking his classmates. Jujutsu Kaisen (TV) Episode 12
Desperate to save his friend, Yuji begs Sukuna to heal Junpei. Sukuna and Mahito both mock Yuji, and Sukuna cruelly refuses to help, leading to Junpei’s death in Yuji’s arms. Despite Yuji's attempts to reach Junpei's heart and