Momo 'challenge': What actually is the horrifying trend said to be terrorising children?

The horrifying image known as "Momo", now apparently being used to torment and horrify children on messaging apps such as WhatsApp, has once again been the subject of chilling warnings. Police have this week expressed concern once again about the image, which they say is being used by cyber criminals and other malicious actors to add people on messaging services apparently in an attempt to steal from them. But it is a long-running trend that has spread across the internet in a variety of different forms, with a true story that is almost as strange as the often horrifying false stories that are told about it. Where did Momo really come from? Relatively little is known about where the Momo image, name, or story came from. But it is far from supernatural and mostly explicable.