"The Truth of This World" (この世の本当のこと Kono Yo no Hontō no Koto ) refers to the various truths concerned with the Crimson Realm, which are hidden from ordinary society.
These truths include the existence in the Human World of the Crimson Denizens, whose dominant nature humans can never overcome, and the truth that those Denizens are selfishly rampaging in the Human World. Since there is a danger of disorder on a global scale due to these truths being impossible for human society to cope with, unless there is a special reason for it, the Truth of This World is generally not told to humans.
Additionally, as long as an ordinary person doesn't interfere with a Denizen of a Flame Haze, they are unable to perceive the various truths concerned with the Crimson Realm, and even if they did, their realization is difficult to spread to the rest of society due to the circumstances.
In the latter half of the 19th century, a group appeared whose activities were made in order for ordinary people to acknowledge these truths, and they named themselves Révolution. Their ideology was regarded as madness by "normal" Denizens and the Flame Haze, and they were eradicated by the middle of the 20th century.
The Truth of This World was eventually changed due to the creation of the New World Xanadu with the rule "Humans can't be eaten" (人を喰らえない Hito o kurae nai ).