Robin at the Break of Dawn, Vol. 1

Norito and Yoshiomi’s story continues in this sequel to Robin in the Veil of Night!
Detective Norito and his twin brother Shinji, a folklorist, investigate a murderous cult in a remote mountain village. There they rescue another set of twins, Yoshiomi and Miwa, from the cult’s grasp—but a year after their escape, Miwa disappears.
Yoshiomi’s only lead on his comatose brother Miwa’s whereabouts is from a dream, which brings him, Norito, and Norito’s brother Shinji to a coastal town that worships a mermaid deity. As they dig into the town’s practices and beliefs, Yoshiomi realizes that the darkness of the Urabe curse is still casting its shadow on his brother.