Reiger was unaware that after his departure, the people of the Nasalik Underground Tomb were still plotting against him.
He was counting the gains from the previous night, tallying the magic he had acquired from Momonga.
Under his deliberate guidance, Momonga indeed revealed a considerable amount of information about tiered magic and even demonstrated each in front of him.
Thanks to this, Reiger managed to learn those spells, thus achieving part of his goal for this visit.
The reason it was only a part was that the protagonist Momonga was exceedingly cautious, and Reiger had not been able to trick him into revealing all 718 types of magic.
Not to mention the necromancy that Momonga specialized in, Reiger hadn't managed to learn a single spell.
Perhaps out of concern for exposing his undead identity, or maybe to conceal his most proud skills and prevent others from fully grasping his strength, the magic Momonga revealed to Reiger did not include his most adept, necromancy.