Complex sentence: In Aeschylus' Eumenides Pheres is mentioned by the Chorus of Erinyes of Clytemnestra, i.e. her avengers for the mother-blood Orestes spilled by guidance of Apollo.
Split into simpler sentences: In Aeschylus' Eumenides Pheres is mentioned by the Chorus of Erinyes of Clytemnestra. The Erinyes where the avengers for the mother-blood Orestes spilled by ordering of Apollo.