When their mother becomes the first victim in a string of murders, devastated twin witches Hunter and Mercy Goode vow to avenge her death, but it will take more than magic to rein in the ancient mythological monsters who have infected their peaceful town. Twins Hunter and Mercy Goode are direct descendants of the founder of their town of Goodeville. As their ancestors have done before them, it is time for them to learn what it means to be Gatekeepers- the protectors of the Gates to different underworlds, ancient portals between their world and realms where mythology rules and nightmares come to life. When their mother becomes the first victim in a string of murders, the sisters vow to avenge her death. But it will take more than magic to rein in the ancient mythological monsters who have infected their peaceful town. -- adapted from jacket

Series numeration from www.goodreads.com.