We're all vaguely aware of the dark side to the internet, but this case is extraordinary. It targets children, and is responsible for the deaths of a 12 year old Argentinian girl and a 13 year old Belgian boy. More recently, it's told an 8 year old boy to put a knife to his neck.
According to this article from inews:
The Momo challenge is an alleged form of cyberbullying which spreads over the phone messaging service WhatsApp. Children receive threats from an anonymous number associated with a picture of “Momo” – an alien-like woman with bulging eyes, an eerie stretched smile, and straggly hair. Momo allegedly forces children to undertake dangerous tasks – up to and including suicide – by threatening them with violence.