The Rainbow Eyed Devil and His Songbird

Familiar

Familiars- a creature or being who's will is to serve, fulfill orders or protect their masters.

but Douma isn't that

In the beginning, Muzan only had to deal with samurai that would become slayers in later generations that stood with the Ubuyashiki family.

Then came the mikos and shamans had been sought out, to see if there was some way to end the threat of demons outside of the means of beheading them. This was all before the time of the legendary Yoriichi who would form Sun breathing.

Swordsmiths made the swords out of the metal that was most effective and still the Ubuyashiki family had faith in the mikos and shamans.

Till one day, they were able to bind the soul of a slayer to that of a demon. The demon fell under the control of the human till the slayer died.

The demon wouldn't die and another slayer would be there to take the predecessors' place.

Yoriichi would be the one to capture and link his soul with his brother that had been turned into a demon.

Their bond was said to be as legendary as their swordsmanship.

It was said there would never be a bond as powerful as theirs.
—
He had been caught.

Douma smiled through his muzzle as chains dragged him into the circle that held another. The one that had caught him and surely would torture him.

A venomous little thing that stared daggers into him.

He didn't quite want that.

Akaza had been capture not too long ago and had been the reason that led to Douma's capture.

Even now he can see the praise that the slayer Akaza was tied to was giving him after Douma had been muzzled and his arms tied down.

No...

When the ritual started, an ancient magic that forced a string from him and another from the slayer before him, Douma raised his head.

...he doesn't want to be tied down like this.

So, he bit through his muzzle and laughed, frost leaving his mouth that poison was leaving from.

—
He doesn't wince at the pain he feels, but he acknowledged it.

He had torn through the barriers and slashed through slayers and even a few tamed demons.

Douma's ice clones allowed him to escape but there was a problem.

His string was out and would need to be tied to something or he would most likely wither away at it's exposure.
—
His running stops halted by singing. A lullaby that drew him in like a moth to a flame.

He would find a young woman with blue hair and gorgeous emerald eyes.

A smirk comes to his face when she dances across the dark clearing.

She would have been a delightful snack but he wouldn't dare to waste this chance to be tied down to someone like her.

His eyes caught her gaze and like a spell, she fell into his arms, into a dance. A waltz that would last till the early hours of the morning.

She was nearly slumbering in his arms when the string of his soul finally found hers. A neat little bow and then a tight knot sealed their lives together.

Douma of the Uppermoons was free of the slayers and now was tied to a woman who would be his songbird.

—
Her name was Kotoha and she was his wife.

Or that's what he cheekily told the slayers, the Flame Hashira and the one he was meant to be bound to, and then Akaza who had his foot on his throat.

"You have bound yourself with her?" Douma nods before smirking.

"And in marriage Akaza-dono. We are actually expecting now! When she passes, my goal is to be the guardian of the clan we make together."

That had the slayers foaming at the mouth but he didn't much care.

Kotoha was a lovely woman to be tied with.

Douma would learn his emotions and have a lovely life with Kotoha. Muzan would visit and the slayers would only come by to threat him once when he ate too many women near by.

Not a straight forward Familiar AU but I hope you enjoyed it!