A player must reach level 15 in Project Slayers before gaining access to the Demon class. You can do this by repeatedly killing the Zuko boss or by grinding quests. He’s part of the Somis questline and can be found in the first village you visit.
Finding Muzan should be your next objective. The only time you’ll see him is at night, and he appears in unpredictable places. His precise location can be revealed by purchasing a Gamepass, but you can also find him by searching the map with the help of the video below.
To complete Muzan’s quest, you must first find five Blue Spider Flower Lillies and then return to him with Doctor Higoshima. You’ll just have to run around and hope to stumble upon some Blue Spider Lillies, as they appear at random locations across the map (including in towns).
If you walk past the Zuko boss away from town and to the right, you’ll find a wooden ramp that leads to Doctor Higoshima’s home. Using the doctor’s assistance, pick him up and bring him to Muzan. On a computer, you can access the “carry” function by pressing the “H” key; on a mobile device, you can do the same by tapping and holding with two fingers.
Unless you found the lillies the same night, Muzan is probably not going to be in the same place, so you’ll have to go looking for him again.