Ou-sama Game: Shuukyoku

Nobuaki Kanazawa is a high schooler. One day, he receives an email with an order from someone calling themselves "king". At first, the orders were normal. However, they started to escalate as time passed. Anyone not following the order was to be "punished" with "death"... The "King's Game" with their lives on the line has started!!! What awaits Nobuaki, who's facing against the invisible "king"!?
(Source: Coolmic)

Side story of Ousama Game; takes place around the same time as Ousama Game: Shuukyoku.

While Nobuaki Kanazawa and Natsuko Honda struggle to escape the curse of the King's game, far away, two schoolgirls in search of thrills break into an abandoned school at night. On the wall, in the notebooks, they find messages of hatred and anger... One of them particularly intrigues them: "Who is the King?" A year earlier, it was in this high school that Natsuko had her first experience of the cruel survival game! At that time, she only thought of one thing: to save her friends, and especially Kentaro, the boy she loved... But good intentions are no match for the King's cruelty... How far will Natsuko and Kentaro go to survive