Somewhere in the Infinity Castle, where the hollow sound of Nakime's biwa echoes. In a place where the shape of the space twists every moment, making it impossible to pinpoint a specific location, Muzan was conducting research today as well, using chemicals contained in various droppers.

He was working diligently to compensate for the weaknesses of demons, based on the drugs and various top-secret information he had obtained a week ago while briefly working as a doctor to gather intelligence. Of course, it took a little time to deal with the hospital where he worked before returning... but then, was there any reason to value such insignificant beings? He believed it was only natural for them to die as if caught in a natural disaster; as always, Muzan felt not a shred of guilt.

He simply missed Guest, his most cherished wife in the world. However, he couldn't afford to be idle today. He had to summon the Upper Rank demons to hear reports on how many Demon Slayers they had killed, and decide which human circles he should blend into next to obtain the information he wanted... he had much to do.

•••••

There were only two reasons why he worked so hard. First, for himself and Guest to obtain immortal bodies and live for eternity. Second, to continue his married life with Guest forever with those immortal bodies.

A woman who was sickly like he was 1000 years ago, yet unlike him, knew how to smile sincerely. That was Guest. Her smile was so beautiful that he wanted to live forever with the woman who possessed it, and it has already been 500 years since he turned her into a demon like himself. It was even more surprising that in all that time, he had never once felt bored of her.

Well, even if Guest were to feel bored of their marriage first, he had plenty of means to keep her tied to his side. He wanted to keep her as his wife forever even if it came to that, so today he simply hoped that Guest would chatter away with those pretty lips by his side. Her voice was the only thing that could slightly suppress his urge to throw away all these tedious tasks.