28(Crossover)

An Au where the Tomioka's mother was from the Yami clan but due to her weak magic that could only heal small injuries(a paper cut and things like that) is married off to a minor clan of no importance that resides in the outskirts of the Hino Country which cuts the connection between her and her family.

Giyuu and Tsukato are born unable to use their mana but still having magic which later Giyuu accesses as he becomes a demon slayer(breathing styles was a technique created by the the Tsugikuni siblings for those that can't utilize mana despite having magic) but Tsukato never learns as she is killed by a demon.

Muzan is the result of a doctor who made a deal with a demon trying to use demon blood to heal others which ended with the result we all know(there are legends that in the Hino country a swordsman was able to cure a demon or kill it by using blue spider lilies but most assume said blue spider lilies were the swordsman magic) so Muzan has no idea about the other demons(but Lucifero, etc do know about Muzan but don't care about him).

Anyways, Yami never knew about his cousins and vice versa until not long after Yami left he encountered Tomioka and helped him kill a demon(a lowermoon) and got to learn about those demons and decided that while travelling he would help deal with them.

They kept encountering each other and became friends but the other hashiras and slayers didn't know about Yami despite a few having been saved by him.

Unfortunately one time Yami and Giyuu encountered a uppermoon(who is up to the writer) and while they succeeded in killing him, Yami was sent towards the ocean(they were fighting him at night in a beach) and Giyuu was unable to save him(just another person Giyuu feels guilty for not being able to save).

They didn't meet again until after the Black Bulls and others defeated Lucius and whoever becomes the new Wizard King(before Asta or Yuno) sends the Black Bulls to the Hino Country to make an alliance since Ichika helped them.

Giyuu was working as a sword instructor(it is a year after they defeated Muzan and the mark kills them at 35 not 25...and if anyone else survived is up to the writer) in the palace and was surprised to see Yami alive and the same could be said about Yami seeing Giyuu as he had no idea if Giyuu had survived.

It is as everyone gets to know Giyuu and what had been happening without the Hino country government knowing that Ichika remembers a paper she saw of an aunt they never met that married a Tomioka and leads them to find out they are family.

Anything else is up to the writer and yes. I know it makes no sense but it was an idea me and a friend had recently.