What is the source of the allusion in the sentence below?
Forced to choose between Scylla and Charybdis, Zachary threw up his hands in dismay.

 - Greek mythology
 - modern history
The source of the allusion Scylla and Charybdis is Greek mythology.
In Greek mythology, Scylla and Charybdis were two sea monsters located on either side of a narrow strait in the Mediterranean Sea.
The allusion Scylla and Charybdis means a pair of distasteful alternatives.