The Tale of the Seven Lords

Overview
The Tale of the Seven Lords (often shortened as TSL) is a series of fantasy novels written by a human named Christopher Peugeot and has 138 volumes, according to Leviathan. He also states that 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. Leviathan thinks the 1990's theatrical version was an "utter disaster" due to the fact that they added characters who weren't in the original manuscript. In contrast, he praises the 2015 version. He also says that many people like the oldest lord the best, but his favorite is the third lord.

In the Devilgram The Battle King, it is mentioned that the card game "Magic: The Gate" that characters are playing is a part of the TSL franchise.

The brothers and MC perform a musical version of the story in the event Henry and the Seven Lords, directed by Simeon.

Story
The protagonist is a regular human knight named Henry, and the story seems to focus on him dealing with 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.

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). It's also mentioned that the seven lords belong to a different species than Henry, who's (assumed to be) a human.

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 misfortunes throughout 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 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.

Character Inspiration
As Luke comments in a chat with Simeon, all the lords are based on the seven demon brothers, while Henry is correlated with MC.
 * The Lord of Corruption is based on Lucifer.
 * The Lord of Fools is based on Mammon.
 * The Lord of Shadow is based on Leviathan, which is coincidentally also his favorite character.
 * The Lord of Masks is based on Satan, do you think Satan is hiding something from us then?
 * The Lord of Lechery is based on Asmodeus.
 * The Lord of Flies is based on Beelzebub, which is a transliteration of his name from Hebrew.
 * The Lord of Emptiness is based on Belphegor.

Trivia

 * The Chapter G Nightmare references TSL with the dialogue: "This should be of some assistance." With those words, Orbus entrusted the light of hope to the princess. — The Tale of the Seven Lords: Volume 7
 * It's title was originally "Tale of The Seven Kings" but it was later changed into "Tale of the Seven Lords"