The Gwyllion are Welsh female fairies, who haunted lonely roads and led unwitting travellers astray.
Gwyllion are ugly old women usually described as wearing ash colored clothing with an oblong four-pointed hat, and carrying a pot in one hand. They also are known by their disturbing laughter and their cries of "Wow up!" This is an English form of a Welsh cry of distress) ' Wwb!' or 'Ww-bwb! [Pronounced Wooboob].
Gwyllion live in mountainous areas and love to mislead travellers or scare them.
Many times Gwyllion will just bother the traveller or possibly frighten them sitting on either side of a mountain path and following him with their eyes. However, if a Gwyllion in disguise manage to enter your home, it is recommended to treat her right and she will leave by herself without doing harm. The Gwyllion is repelled by metal knives, and a flash of a knife is usually enough to send a Gwyllion away.
One particular gwyllion was the Old Woman of Llanhyddel Mountain, an old hag with a four-cornered hat, wearing garments of ash-colour and an apron thrown over her shoulder. She always seem to carry a wooden milk-pail, and often shouted a characteristic call, "Wow-up!". She was impossible to catch, and any who saw her were sure to lose their way.
Another famous gwyllion is the Old Woman of the Mountain who haunts Lanhyddel Mountain in Monmouthshire.
The Minor Traditions of British Mythology by Lewis Spense.