is jujutsu kaisen episode 12 filler?

Junpei attacks his school, and Yuji arrives to stop him. Yuji tries to reach Junpei with his fists and his words, promising to bring him to Jujutsu High where things can be different.

For a moment, it seems to work. Then Mahito descends and transfigures Junpei into a twisted monster right in front of Yuji.

Yuji begs Sukuna to help. Sukuna laughs.

Junpei dies. The scene defines Mahito as Yuji's most personal enemy and establishes the emotional stakes that will carry through to Shibuya and beyond.