The Prophecy

"In the seventh month of 1999, a great king of terror will descend."

The prophecy of Nostradamus was revealed in the 122nd Chapter of the Chainsaw Man manga. It foresees the extinction of humanity in July 1999. It is the most important foreshadowed event to date.

According to Fami, Yoru is the only individual possibly capable of stopping it.

Story
The first mention of the prophecy's existence comes from Hirofumi Yoshida, who believes Fami to be involved with it in some way and met her for questioning. He further reveals that Public Safety made 30 convicts make a contract with the Future Devil under the terms of release from custody. The convicts received the knowledge of the dates of their deaths for an unknown price, and it was confirmed that 23 out of the 30 would indeed die in July 1999.

Fami finishes the exposition by revealing that the 7 remaining convicts will die this week and that 40 seconds ago a primal fear, the Falling Devil, appeared on Earth and that devil will be the first of many to cast the world into the abyss of terror.

The aforementioned devil begins a mass slaughter of Tokyo's population casting countless people into Hell where they'll be devoured while also murdering any devil hunter in sight. She chooses the war horseman, Yoru alongside Asa as her next targets and plans to "serve" them as the main dish. However after being "served" they escape Hell with the help of Chainsaw Man and a mysterious benefactor. Because of the harsh terms set by the Falling Devil, other devils desperately attempt to hunt them down causing severe collateral damage and deaths through the city.

Fami is revealed to be somewhat orchestrating some of the events leading up to the prophecy. However, it's revealed that rather than her being complicit with its events, as she believes that they are fated to happen, she instead wants to subvert the prophecy's meaning to ensure humanity's survival.

Origin
The prophecy finds its origin in the real world. Nostradamus' original writings were written in archaic French and is notoriously vague and difficult to translate. As such, there are many translations of this particular prophecy (officially named Quatrain X.72). The following translation comes from the book "The Secrets of Nostradamus" by David Ovason:

"The year one thousand nine hundreds ninety nine seven months''

From the sky will come a great King of alarm

To bring back to life the great King of Angoulmois.

''Before after Mars to reign by good fortune."

Trivia

 * While this line does not appear in the manga, Mars is the Roman god of war. In this context, he is most likely represented by Yoru.
 * The King of Angoulmois in the original prophecy referred to Ghengis Khan, as "Angoulmois" is a near-anagram of "Mongolians." Whether or not there is a character that parallels this is yet to be seen.
 * The text on the back cover of Volume 12 forsees that Yoru will unleash a great calamity upon the entire world. Very likely related to this prophecy.