Black Valley by John Washbourne

Black Valley is The Wicker Man meets Silent Hill: a dark fantasy novel drawing on ancient Celtic Myth and grounded in post-industrial North Wales. 1918: The Manager of a Welsh slate quarry at Cwm Du (Black Valley) pushes ahead with plans to open up a new slate face in a natural cavern in the mountains, despite the protests of his workers and the wise woman of the village, the Druidess Myfanwy. The resulting explosion not only unleashes underground water to flood the quarry killing hundreds of quarrymen, it also brings with it the ancient race from the Otherworld below: Gwyllion. To save the world from the wrath of the Gwyllion and the pestilence they carry, Myfanwy seals Cwm Du from the outside world: Cwm Du becomes a forgotten lost valley shrouded in myth, guarded by her daughter, Becca, the new Druidess. 2020: a hundred years later: Becca's only female descendant, Laura, is bequeathed a house in the village of Cwm Du, beside the now flooded quarry. Laura travels to Cwm Du with her boyfriend Jimmy. Laura and Jimmy enter Cwm Du to find a place where the Gwyllion are suddenly returning, threatening the inhabitants and the outside world. Laura's unexpected role is to become the next Druidess, but can she do so and save the world when the Gwyllion are at her door? "Nothing can hold back the Otherworld: Except You"