The moment Eren finally breaks occurs several episodes before he orders the Titans to devastate Marley’s Kingdom. The episode “From You 2000 Years Ago” provides one final Titans history lesson as Eren’s brother, Zekem realizes that Eren quietly played him. Throughout the fourth season, Eren is tied to a prophecy about a cataclysmic event known simply as “Rumbling”. With Zeke foiled, Eren leans into his role as the trigger for Armageddon, vowing to destroy anyone outside of Paradis.
Although Eren loses control of himself even more violently as the season progresses, “From You, 2000 Years Ago” establishes that he is already too far gone. Eren has been manipulated by Zeke for a long time, so seeing him outwit his brother brings a brief sense of triumph, but it quickly gives way to the horror of what Eren has become. “Attack on Titan” is about dire consequences, and the fact that Zeke’s plans for Eren backfire and push Eren into evilness provides the kind of tragic poetry the series thrives on.