In the end, the cage didn't open originally because Sebastian did not read the ceremony rules. Once he read the ceremony rules, the door to the cage opened to him in a way that left him feeling ashamed.
His reaction on waking up was like a beast, like the Mana Lords expected. Instead of calming down and trying to understand his circumstances, the first thought that came to him was ramming his way out of this cage.
He felt embarrassed just imagining the scenario in his head.
"I feel worse for those who cannot read".
At this moment, he was back in his Megalodon form. He didn't know how the Mana Lords did it, but he guessed that they simply killed the Mana Lord doppelganger that he formed on the spot, revealing his real body.
As for the doppelganger transformation ability that he used on both Strong and Tungsten, it was instinctive, Sebastian used the ability after being pressured by the threat of death associated with being exposed.
