In GOOD NIGHT WORLD, the Black Bird is an end-game content inside the immersive online game PLANET that starts threatening the lives of players. The Black Bird offers 300 million yen to the player who can defeat it. In the greed of grabbing that money, many players start looking for the Black Bird.

The Black Bird’s true identity is Good Night World, an artificial intelligence created by Kojiro. Leon calls it Sasumata, the sub-master of the Granada Guild and his only friend in life. Sasumata, despite being an AI, is smarter than anyone and filled with rich emotions. The Black Bird consumes emotions, enabling it to move and grow.