Nightbringer is a sequel game to the original Obey Me!, in that the Main Character (aka "MC") — the protagonist that players experience the game through — is the same individual from the original Obey Me! storyline. While playing the original game and knowing the plot of the Main Story from said game does enhance the story experience of Nightbringer, new players are also able to pick up the game without having played the original.

The story of Nightbringer begins with MC receiving a mysterious text from an unknown person, who then transports them to the Devildom, and to a time in the past just after the Great Celestial War. At this time, the seven brothers have only recently fallen from the Celestial Realm, and the world of demons is an unstable, dangerous place for humans. Because of this, MC has little choice but to tell everyone they are a fellow demon: a difficult lie to maintain around a distrusting Lucifer. While neither he nor his brothers remember MC, they will be the key to helping MC return to their present timeline.

[from Obey Me Wiki]