Lucifer convinces Mammon by explaining that he’d be publicly recognised as the number two of the demon brothers if he goes - if not, he’d give up the position to Satan or Asmo. Mammon agrees and expresses how 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, but Lucifer wants to start right away. That makes Mammon doubt his intentions as he suspects that Lucifer went along with the plan only 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” (which was 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.