Complex sentence: The Graeae were daughters of Phorcys, one aspect of the "old man of the sea," and Ceto, and thus were among the Phorcydes, all of which were archaic beings either of the sea or chthonic deities.
Split into simpler sentences: The Graeae were daughters of Phorcys, one aspect of the "old man of the sea," and Ceto. Thus, they were among the Phorcydes, all of which were archaic beings either of the sea or chthonic deities.