Sekiryuutei's Battle For Humanity, Begins At Sea (DxD x Battleship 2012)

[A slight crossover between DxD, Battleship 2012, FGO and Azur Lane.]

Issei Hyoudou, the main protagonist of DxD, got betrayed by the woman who loves him are: Akeno, Koneko, Xenovia, Irina, Rossweisse and Ravel. Those who remained faithful to him are: Rias Gremory, Asia Argento and Ingvild Leviathan. Other girls such as Gabriel, Amaterasu, Penemue, Artemis, Athena and Tiamat takes their time to confess their love to him.

When Issei quits in the peerage and becomes a human-dragon hybrid, he joins the naval training and graduated as a top student of Naval academy Etajima. He then participated in the RIMPAC Naval Exercise in the coast of Hawaii, even Rias, Asia and Ingvild participates on joining this exercise as an observer. However, when the outlanders from outer space arrived on earth, they began to conquer the seas and destroy their naval power. Issei along with his lovers has two options: To Fight Back or Die.