Which episode in season 3 showing Eren having to seal a rock with hardening? 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?
In 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