As far as i remember the manga starts off in a train or in a metro, and it's completely dark, if i remember correctly the train is full of cockroaches or something like that. Not long after that protagonist finds a heroine. Most of the manga takes place in complete darkness, and if I remember correctly they stay in the darkness because it could uncover them but I could be wrong about this part.
I haven't been able to find this manga for a while, so I don't remember it that well anymore. Thank you for helping me find it.

Distant Sky?

dragon head?