The Tale of the Seven Lords

The Tale of the Seven Lord (often shortened as TSL) is a series of fantasy novels written by a human named Christopher Peugeot. It has 138 volumes and according to Leviathan, it's the most widely-read fantasy series in the world.

TSL has theatrical versions, an animated series, feature films and has been translated into 182 different languages.

Story
The protagonist is called Henry, and the story seems to focus on him dealing and helping the title characters. The seven lords are brothers, the oldest being the Lord of Corruption, who's always plotting and planning in secret. The second oldest is the Lord of Fools who'd do anything for money. The third brother is the Lord of Shadow, a brooding recluse. The fourth oldest is called the Lord of Masks, who acts as a high-status, upstanding member of society, but in truth he's inhumane and cruel. The fifth brother whose name is the Lord of Lechery has a dirty mind and only thinks about seducing others. The sixth oldest is the Lord of Flies and he only has one thing on his mind - food. Lastly, the seventh brother is called the Lord of Emptiness, who's hard to understand or read. It's also mentioned that the seven lords belong to a different species than Henry, who's (assumed to be) a human.

Not much is known about the plot of the series, other than a few parts that are mentioned through the gameplay (most of them being out of context).

In lesson 3-10, MC watches the movie series of the books, along with Mammon and Beelzebub. Mammon is shown to feel empathy towards the Lord of Fools, as the second oldest experiences many misfortunate events through the story. In Volume 2, the second lord reunites with his is former lover, Geldie, but he finds her frozen in ice and Mammon hopes the lord will find a way to save her life.

Then in Volume 5, the supply lines of the Lord of Flies gets cut off after his land is invaded by hostile forces. The Lord of Fools sends food supply to him, but the food spoils because of the rain, causing a disease spreading across the Lord of Flies' land. This upsets Mammon even further, as he feels for The Lord of the Fools, who only wanted to help his brother.

In Volume 7, the Lord of Emptiness leads a sudden rebellion against the oldest lord in year 825 of the ancient era. The Lord of Corruption captures and imprisons him within the Tower of Shadow, which is in the farthest corner of the world. Although the other brothers lament his fate, they also realize there's nothing they can do as they fear the wrath of the Lord of Corruption more than anything else. Mammon comments that even though the Lord of Emptiness deserves some blame, his brothers still shouldn't abandon him like that.

Trivia

 * All the lords are heavily based on the seven demon brothers, while Henry is based on MC.
 * Leviathan thinks the 1990s thetrical version was an "utter disaster" due to the fact that they added characters who weren't in the original manuscript.
 * In contrast, Leviathan praises the 2015 version.
 * Leviathan says that many people like the oldest lord the best, but his favourite is the third lord.
 * He likes Henry too, to the point that he names all of his pets after him, and admits he'd like to cosplay as Henry.