Hollow Regalia, Vol. 6 (novel)

Three years after the ploutonion closed, the world that narrowly escaped destruction has steadily rebuilt itself. Though they saved everyone, Yahiro and Iroha's whereabouts are unknown. One day, peace is disrupted when Gallery Berith receives information that moujuu have reappeared. Meanwhile, on an unnamed island in the south, Yahiro and Iroha have been kicking back and taking it easy. A mysterious girl washes up on their island, and Iroha has her guard up--the girl seems to know quite a bit about Yahrio and the reality of their world. Not to mention the girl is being targeted by monsters. Not the moujuu, but a new creature entirely.