In the dark world of Hellsing, Alucard and Seras got back from his slaughter against some vampire scum. Integra, ever so pleased, huffs her cigar, her butler, Walter, stands behind her, smiling, secretly proud that this little rag-tag team has turned into a real family. Then, they all, somehow, appeared in the world of Demon Slayer

Integra: “Seras!? Alucard!? Walter?! Oh, good, you three are here. Ahem, let us gather information.”

Seras: “Oh bollocks, I was hoping to have some alone time with Vernedead, but alas, that didn’t seem to be able to happen.”

Walter: “No need to worry, Ms. Victoria, I am more than certain Vernedead will wait for you to get back.”

Seras, with a flustered tone: “Oh, y-you think so?”

Alucard, arrogant bastard he is: “Hm, that human really has taken a special… allure to you, Police Girl.”

Seras: “Ugh, my name isn’t Police Girl, Master.”

Now, you can continue as one of the Hellsing members, a demon slayer, a demon, a normal human, or your own OC. Have fun!