Helen Susan Swift 
Dark Mountain [EPUB ebook] 

In the West Highlands of Scotland, after the end of World War One, Brenda and her friends are preparing for an expedition of a lifetime: climbing An Cailleach, also known as The Witch.
But even before they reach the base of the mountain, they realize that something is wrong. Strange apparitions, even stranger locals and ancient superstitions tell them of the dangerous path they have chosen.
When things take a turn for the worse, will friendship be more important than survival?