Japan has been ravaged by attacks from devils and fiends, and Asa/Yoru have recently escaped from a fight they couldn't win. After much deliberation, Yoru convinces Asa to seduce a person to help her get experience (to prove a point) and turn him into a weapon afterwards

After becoming the host of Yoru, the War Devil, the Asa Mitaka will have to help her take revenge on Chainsaw Man. Yoru wants to kill Chainsaw Man (Pochita/Denji) primarily to reclaim her pride and power as the War Devil, stemming from a past humiliation where he almost ate her and erased her. As the War Devil, she seeks to regain the powers he took from her and to prove her supremacy over him, a goal that keeps her motivated by a deep hatred and desire for revenge.

However, Asa is proving to not be a very competent host for Yoru thus far.