In Kancolle, the shipgirls(girls based on irl warships and wearing the same armaments) are fighting a seemingly endless war against the Abyssals(the ones you see here, there's many of them). Not sure if it changed now as i've been away from kancolle for many years, but basically nothing is known about their existance, how they come to live etc. As such there are many theories around and the most common was that Abyssals are born from the ships that die full of regrets or something along that. I imagine this work includes the author's view on the matter on some level along with the 'it never ended' angle