Afterword:
I actually finished the draft of this R-18 chapter before Side story 3. However, I felt that releasing this before Side Story 3 would be weird, so I only finished drawing this chapter after Side Story 3.
Basicacally, the relationship and chronological order between this chapter and the previous six is as follows:
Side Story 1 takes place after the end. It's a Shoukaku-route story that's connected to the next installment.
Side Story 2 serves as a prologue. It's mostly about Nagato and Houshou. I actually regret striking out too much of Mutsu's storyline; I will do another chapter solely focused on Mutsu to make up for it, since there is a lot of Mutsu-related storyline remaining.
Side Story 3 also counts as a prologue, but the last half is focused on the Shoukaku-route material that's coming up in the next installment.
And this R-18 chapter supplements the storyline between Nagato and Houshou. To be honest, the original reason why I drew this chapter was because I felt most people who read my manga would want to see more H-scenes between Nagato and Houshou, so I didn't put much thought into the plot at first. I added more material plot-wise afterwards, especially after I started drawing the next installment and wanted to use this chapter to explore some of Nagato and Houshou's inner thoughts.
If you have other questions, but can't leave comments on Pixiv then you can ask me on Plurk, Weibo, or Twitter; I will try my best to answer you.
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Peace out.