Owarimonogatari End Tale. Part 3

From the best selling novel series comes the latest book of the final season of the MONOGATARI Series. During his third year in high school, Koyomi Araragi is introduced to a transfer student named Ougi Oshino. Ougi tells Koyomi that there is something odd about Naoestu High School... a secret room on that's not on the map. What will Koyomi and Ougi find in this hidden room?