room no. 9

BL Visual novel by Parade.
"Best friends Daichi and Seiji set out on an almost too-good-to-be-true summer vacation, but it quickly turns into a nightmare...

Instead of the island paradise they were expecting, they find themselves trapped in a strange room and forced to participate in an incomprehensible "experiment". They'll have to play along to have any hope of escaping with their lives, but that means hurting each other and possibly destroying their friendship—and their minds—in the process."
(Steam description)