Black lightning room was kinda weird. Break the ceiling in front of the lightning door. Dunno what that meant.
Viewing post in The Case of the Dark Strangler comments
spoilers
It shows the personal motive behind the Dark Strangler's actions. Someone close to him was also a black lightning victim and the germination took them too, though it's unclear what happened to them after that. The Dark Strangler was killing the black lightning victims not only to stop the spread of the germination, but also to prevent anyone else from losing someone they care about to it.