This is Kuchisake-Onna, the Yokai that asks you if she is pretty.
She comes up with a facemask and asks you if she is pretty.
If you say yes, she will uncover her face and you can see her gory ripped-out mouth and she will ask you if she is pretty agan.
If you say no, she will get a pair of ssisscors (idk how to write this word) and will cut your mouth the same way as hers.
If you say yes, but with visible anxiety and fear, she will take you to your home and then stab you with the same sword that her husband cut her mouth with.
If you say yes in a calm and honest tune, she will give you a bloodied diamond and then leave.
But the best thing to do is, when she meets you with the mask on and asks you to rate her face, say that you have other things to do, and you will easly avoid her.
If I met her, I would not be surprised by her face.
I have seen real gore that is way worse.
This is just an clarification of the legend.