Appearance of demons
How does the appearance of people who become demons change? So, Muzan turns someone into a demon, and who decides what he'll look like? Does Muzan himself? Like, you'll just get weird square sparkles on your cheeks, or your eyes will turn into white tennis balls, or you'll grow a horn on your forehead... Or is it random, a matter of luck? Why are some demons barely distinguishable from humans in appearance (unless you look inside their mouths), while others look like the spawn of Cthulhu?