The facility was destroyed during the Blue Night, leading to the formation of the Exorcist Cram School. Mephisto Pheles stored Section 13 in a separate dimension, preserving it for investigation ...

Satan escapes Section 13, and to stop his reign of terror, Yuri makes the ultimate sacrifice, offering to stay with him ...

Another important element in Blue Exorcist: The Blue Night Saga episode 2 was Rin Okumura discovering the truth of how Mephisto was creating several demon king clones to get Lucifer a new body.

Still, Mephisto knocks him down when that happens. Blue Exorcist: The Blue Night Saga episode 7 will probably continue with Rin Okumura's time travel and feature more of Mephisto and the events ...

It's kind of notable to see Lucifer and his fellow divinities swearing fealty to Satan so readily, but it's a mundane scene. And the moment when Mephisto Pheles refuses to swear fealty is thrown away.