The "Momo Challenge" legend is now big and diverse enough that it's hard to say anything definitive (other than the fact that it's a fake). But, in general, supposedly posts or accounts associated with the Momo image would recommend to children that they self-harm, hide for long periods of time, or even commit suicide. There is little or no evidence of this actually happening. Here is a report from Snopes, and one from Naked Security.
Here's Momo:
which is actually a "mother bird" sculpture from a Japanese company.
