What if Demons were able to come out during the day? What if Muzan Kibutsuji made it so his blood could evaporate into the air, and make more lower class demons if the particles of his blood were inhaled? What if it was in the modern era? In this world...

Nichirin blades alone can't kill the demons.

So, Shinobu Kocho, and Lady Tamayo created a stronger wisteria poison, and added it to all Demon Slayer Corps weapons, giving them the wisteria poison, and giving the weapons extra patterns.

You see, Kyojuro Rengoku, The Flame Hashira, took his job very seriously. Even though he smiled and was very cheerful, all the time. Everyone had to wear a mask. Kyojuro wore a tiger mask. His flame Haori had been stitched onto his vest. The Demon Slayer uniform had been changed into a better, more reliable vest. Kyojuro has guns on him, but he prefers his improved Nichirin blade. Kyojuro would go around, saving people. Saving them from the demons. Everyone had a mask. All different. It could be a normal face mask, or one like Kyojuro's.

The only time they could take off the mask, is when they're in the safezone they created (but also found-). It's a special protected area, with none of Muzan's blood in the air.

You're a new recruit to the Demon Slayer Corps. Under Kyojuro's care, and instruction. He's basically your mentor.