Here we see Tomoe Gozen fulfil her obligation to the Oni race by helping them reproduce, afterall, only an Oni should be able to impregnate and Oni. Tomoe Gozen has been captured by Oni who are mad that she once produced a human child, decide to punish her by impregnating her to make her give birth to Oni. Tomoe Gozen who still loves her husband is furious such a thing is happening to her, or that the Oni would violate her for such a reason. But it is too late as she eventually gives birth to her new son. As for why she is in her swimsuit, because it makes her look more sexy.

Artist: https://www.pixiv.net/en/users/353422