It's a boss weapon. You have to kill Margott, who (I believe) is up at the Capital. I think she's one of the last bosses in the game, though.

Morgott is in the city under the Erdtree. It is a harder version of the Margit fight.

Progressing the game

Yes its

I'm not sure if it scales the damage, but it scales the blood loss build up, like reduvia (dagger) arcane stat

Reduvias blood loss does not scale with arcane. Only AR is affected by weapon scaling in this game

kodaxmax, that's demonstrably false. A weapon with arcane scaling and bleed will increase blood loss as arcane goes up, this weapon specifically goes from ~33 to ~90 between 10 and 60 arcane. It's not like previous games where luck scaled all bleed damage, arcane simply scales the effect buildup when the weapon has arcane scaling.

I assume you'd have to wait until NG+, but if you have the stats to do so, you SHOULD be able to

I'm planning on dual wielding this with the Bloodhound Fang for crazy bleed potential.

I was planning on duel wielding this with the bloodfang for double the bleed

I'm disappointed in the sword so far, worse damage, worse scaling, ridiculous stat requirements when compared to bloodhound fang. The awful weapon art is the icing that makes me say just take 2 fangs for dual great curve builds.

does replicating the remembrance from a walking shrine can't buy the same weapon?

I savefile duped it and used it on NG, you could also have a friend just give you theirs if they don't want it. And yes, the combo is as insane as it sounds. Winblades on steroids.