Hell Is Dark with No Flowers, Vol. 4 (light novel)

Shiroshi and Seiji receive a letter of challenge from Ibara and head to a certain sleeper train to defeat him and put an end to the competition over Hell. There are six passengers on the luxurious train, and soon after it leaves the station, a series of murders kicks off. Who is the executioner hidden among them? As he faces off against Ibara, Shiroshi must take the role of detective and solve the mystery before the night is out!