'Most lighthouses are a promise of sanctuary. My lighthouse is a threat.' 

The beam from Guinevere's lighthouse fends off the horrors lurking in the ocean surrounding the isolated Ynys Halen. Guin has one job: keep the light on.

So, when Guin slips up, monsters are free to ravage the island and terror ensues for all who live there. No longer confident that the lighthouse can protect them, the islanders are all too keen to return to the ritual sacrifices that once sated the sea. As punishment, first in the firing line is Guin's more-than-a-friend, Catrin Morgan. In a month, Catrin will be thrown to the ocean to be devoured by the secrets beneath the waves. 

But Guin won't let that happen. She can't. Because it's not just Catrin's life at stake; it will mean the floodgates for future sacrifices will be flung open, and anyone could be forced to pay the cost. 

To succeed against the island's rapidly forming cult, Guin and the eerie Morgan sisters must take a stand, using the lighthouse as their fortress. And as the community descends into depravity, the women must unearth the pestilence beneath the waves and determine who the real enemy is.

Whatever you do, don't let the light go out...