~The secret wife~ {Muzan X Tanjiro}

~Infinity castle~

As all the hashira and some demon slayers, including nezuko, inosuke, genya, zenitsu, and kanao, are exploring the forest, they encounter a sort of portal deep within the mountains.All hashiras jump inside with the others following from behind.Around them is a traditional furnished room with the basic entry way necessities.Gyomei leads the way into the other room realising it's a living room.

However...

There is someone on a stand holding a stringed instrument known to be...a biwu

The string was played, and all hashiras and demon slayers ended up in another room, a more fashioned room than the last it had a warm feeling as the fireplace was glowing a deep orange with a mix of red and yellow.

Mitsuri: "Hey guys....is it just me or...is that a demon over there...." she whispers, pointing to the couch not facing them as a red long-haired person sat on the couch sleeping in a baggy shirt wearing shorts.

Sanemi: "Hey, the demons sleeping... this is the perfect chance to cut off its head!!"

Tengen: "Last time I checked, I'm sure demons don't need to sleep...so why is this one sleeping..."

Mitsuri: "kya~! It looks so cute!! Do we have to kill it..?"

Obanai: "Mitsuri, it's our job... we have to kill this demon, or else it'll possibly harm others, including us."

Sanemi: "Let's kill it now!"

They sneak up on the demon, raising their nichirin blades to sever the head of the demon.

...

The string of the same biwu was played...

Muichiro: "Is it just me...or are we not in the same room as before..."

Uzui: "it's like these rooms are shifting, possibly moving us to different locations..."

Shinobu: "If that's the cause, then there would maybe be a demon controlling the infinity castle...it may also be the cause of that string noise."

Gyomei: "Then let's search for this demon..."