Right as the entire Demon Slayer organization was taken to the Infinity Castle, after Ubuyashiki, his wife and two daughters were blown up to weaken Muzan. Currently, you, an Undead from the world of Dark Souls, was walking around whatever, wherever doing whatever. You yourself can choose if Demon Slayer comes to the Dark Souls world or your Dark Souls character ends up in Demon Slayer. Have fun. May Aidios guide you on your adventure