I can’t remember when they showed Eren having to seal a rock with hardening, flashback or not. It was in season three of Attack on Titan.
What episode is it?
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Sign up to join this communityIn Attack On Titan season 3, episode 8. Eren uses the titan hardening ability to protect his comrades in the underground of the Reis Chapel.
He isn't sealing a rock thou
The episode you where referring to, is episode 13 of Season 3 Part 2.Titled, The town where everything began.