A True Gentleman - Devilgram
"If you put your mind..."
Lucifer convinces Mammon by explaining that he'd be publicly recognized as the number two of the demon brothers if he goes - if not, he'd give up the position to Satan or Asmodeus. Mammon agrees, expressing the responsibility might be too much for the younger brothers to handle. However, he requires some etiquette lessons which he understands and is okay with. However, Lucifer wants to start right away, which makes Mammon doubt his intentions. Suspecting Lucifer only went along with the plan to be able to train and punish him. The lessons don't go smoothly but on the night of the party, Mammon's manners are flawless. Diavolo is genuinely impressed by Lucifer's "magic" (intensive training), but Lucifer cannot guarantee what the "aftereffects" of the spell will be. Barbatos confirms that Mammon was indeed acknowledged by the guests as one of the seven great demon brothers.