"he dreamt of wild men who came to him with clubs and their teeth were filed to points and they gathered around him and warned him of their work before they even set about it"
Cylindrical vessel with Mayan sacrificial scene; c. A.D. 600 - 850
Public DomainCylindrical vessel with Mayan sacrificial scene; c. A.D. 600 - 850 - Credit: FA2010

That this is the prelude to a sacrifice is made explicit in a similar dream described towards the end of Cities of the Plain.

Mayan skull with jade-inlayed teeth
Creative Commons Attribution Share AlikeMayan skull with jade-inlayed teeth - Credit: David Dennis
The reference to filed teeth indicates McCarthy had in mind the Maya, who were known to ceremonially file their teeth to sharp points, with the elite classes often inlaying theirs with jade. 

The ancient Maya practiced dental mutilation over a very long span of time, beginning centuries ago and carried out right up until the European intrusions of the sixteenth century. Teeth were filed into points, ground into rectangles and drilled with small holes to permit the insertion of small round pieces of jade or polished iron pyrite (fool's gold). In all, over a hundred different patterns of cross-hatching, circular holes and shape alteration are found among the ancient Maya.

(Dr. Herman Smith, Maya Dental Mutilation)

Although the Maya never took to human sacrifice with the same ferocity as the later Aztecs, the practice was not uncommon. During the Classic Maya period, human victims were thrown into a deep well at Chichén Itzá to bring rain. Sacrifice on a larger scale may have been brought to the Mayans by the Toltecs, who exerted a similar influence upon the Aztecs. At some point, the Mayans moved towards the ceremonial cutting out of the still-beating hearts of their human victims.