A deadly magical curse called sereno is killing people in the arid borderlands around the city of San Citlán. After an encounter with an outlaw in the village of Milpazul, the characters learn the source of the curse is a tlacatecolo—a shape-shifting fiend. Fearing for the folk under her protection, the outlaw directs the characters to the ghost town of Hollow and its abandoned mine, where she believes the creature lurks.