Ao No Exorcist

Demons break into the monastery, targeting Rin. When Rin calls Shiro unworthy as a father, Shiro's hurt emotional state causes him to let his guard down and be possessed by Satan. Shiro gains control at the last moment and sacrifices himself to protect Rin. Upon seeing this, Rin fights back by unlocking his demonic powers with Kurikara, the sacred sword in which his powers are sealed. At Shiro's funeral, Rin meets Mephisto Pheles, who agrees with his wish to become an Exorcist to defeat Satan.