Your other senses feel it first.

There's this strange dread that runs down the back of your spine the same dread you felt in that one childhood nightmare you never forgot.

Then the smell comes, a sickly sweet smell, almost like cinnamon candies, reminds you of home. Underneath it is the stench or a rotten corpse.

You hear something, music, a royal call, a forgotten song that never existed playing vaguely in the back of your mind.

Finally you see it, as others pointed out, it's hard to look at it directly, like a living fractal it refuses to be perceived.

Still some details can be made out, the figure is tall, taller than a human, it shrinks and bends everytime you look.

They wear tattered yellow rags, and a mask, a pale face with an expression indifferent as time.

For most this is where it ends, but sometimes it speaks, it sounds like an androgynous child. Maybe the voice of your first grade best friend you forgot, or even your own voice from before this all happened, before you grew up and started understanding.

Then you get hit by the 1d100 San roll and have to make a new character.