Saikyou Saiaku no Ura Boss, Tenseisha to Tomo ni Hametsu Flag Kaihi wo Mezasu

Schneizel, the second son of the prestigious Arclight family, is exiled after it's discovered he has dark magic. This event sets him on a journey to find the Evil God and demand to know why dark magic is so despised.

But Schneizel has a secret: another personality named Sanada has been living inside him since childhood, and he's taken over! According to Sanada, if things continue as they are, Schneizel is destined to become the "strongest and most evil final boss" and be destroyed.

To break this desperate death flag, Schneizel's fight has begun!

(Source: Takeshobo, translated)