Muzan's visual effects

Did you know that Muzan’s entrance into the Ubuyashiki mansion actually doesn’t have any fog? The entire Demon Slayer Corps headquarters is filled with wisteria flowers, which are used as a repellent and powerful poison against demons. So, the “smoke” we see isn’t really fog — it’s actually Muzan’s skin burning and regenerating instantly, creating that incredible visual effect. FB: Daily Dose of Anime