I always find it fascinating when writers take two characters who barely interact in canon and build a whole dynamic from scratch. With Enmu and Muzan, you've got the ultimate power imbalance on steroids, which seems to be the main playground. A lot of fics explore total devotion from Enmu's side—like a moth to a flame, but the flame is a sadistic, ancient demon who views him as a particularly useful tool. The best ones don't romanticize the abuse inherent in that; they lean into the horror of it, making the 'relationship' a study in psychological degradation. Enmu's dream powers offer a creepy intimacy, a way for Muzan to be worshipped not just in reality but in the subconscious of his followers. It's less about romance and more about absolute, terrifying control.
I've seen some attempts at a more mutual, manipulative partnership, where Enmu uses his position to subtly influence Muzan's plans, but they often feel out of character. The core appeal, for me, lies in that chilling master-servant dynamic. The tension comes from wondering if Enmu's fanatical loyalty will ever crack, or if Muzan will finally discard him when he's no longer useful. It's not a ship I seek out, but when done well, it can be a darkly compelling character study.