I just realized that three episodes were throwbacks
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In "Overture", Lute and Adam find a dead Exorcist, but the head is dead. No one knew who killed her.
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"Scrambled Eggs" it turns out that Velvette had the head and brought it to the meeting. They all looked shocked and Velvette knew that Carmillia had something to do with it, but Carmillia didn't let her get to her. After the meeting, she tells Zestial that she did kill the angel just to protect her daughters.
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It's also brought up in "Hello Rosie" when there's a flashback of how Carmillia killed the Exorcist. I'm guessing that their expressions change when they get killed and their masks are red instead of white.