Trilogy consisting of Agamemnon, The Libation Bearers and The Eumenides (8)
I believe the answer is:
oresteia
(Other definitions for oresteia that I've seen before include "Greek plays" , "Play trilogy by Aeschylus, see a riot (anag.)" , "three tragedies for Aeschylus" , "Aeschylus's play trilogy" .)