Complex sentence: The "Achilleis" is a lost trilogy of plays by the Athenian dramatist Aeschylus that concerned the actions of Achilles at Troy from the death of Patroclus through the ransoming of Hector's body.
Split into simpler sentences: The "Achilleis" is a lost trilogy of plays by the Athenian dramatist Aeschylus. The three plays that make up the trilogy presumably concerned the actions of Achilles at Troy from the death of Patroclus through the ransoming of Hector's body.