Rumors (噂, Uwasa)? are an important plot element in the Persona 2 Duology.
Sumaru City, in the Persona 2 duology, has the ability to create "Rumors," which is the ability to turn words into reality just by spreading them and having the population believe in them. In game, they can be used to unlock new features around Sumaru City, such as creating additional weaponry and medicine stores, spawning demons of the Rumor race to get new items, and expanding the inventory of current stores with advanced medicine or weaponry. The people in Sumaru City use this ability to make the city prosper, ensure good fortune and create loving relationships. Rumormongers receive this information, spread it and turn it into reality.
However, the power has a much more sinister side, which is any event that has a negative influence on humanity, such as those that pertain to world destruction, can become reality due to rumors, regardless of how nonsensical or childish they might sound. Antagonistic factions of the game, such as the Masked Circle and New World Order fabricate these seemingly nonsensical events to turn them into reality by spreading them, allowing them to destroy the world just by spreading these rumors.
In reality, the power is granted by Nyarlathotep to Sumaru City in order to allow him to rig his bet with Philemon in his favor, displaying that humanity desires destruction instead of salvation. He manipulates his servants such as the aforementioned Masked Circle and New World Order into spreading bizarre and nonsensical catastrophic events and turning them into reality, effectively destroying the world via people's thoughts. He did succeed in his plans during the Innocent Sin timeline by engineering the Oracle of Maia, but Philemon averted it by wiping out the memories of the previous timeline from the party. However, Tatsuya Suou refuses to have his memories wiped out, allowing Nyarlathotep to attempt to restore the memories of the Other Side in order to destroy the world again. Fortunately, the plan was thwarted, and Nyarlathotep was banished to parts unknown.
Persona 2: Innocent Sin
The Oracle of Maia is a bogus prophecy that is dictated by Nyarlathotep to Tatsuya Sudou, which he spread to Akinari Kashihara to have him write it in the In Lak'ech and is one of the rumors that pertain to world destruction. The Last Battalion is another one of the destructive rumors that is spread via In Lak'ech during the events of the game, which involves the revival of Hitler as a commander of the super-advanced Nazi army. Nyarlathotep's plan is eventually successful due to him manipulating Maya Okamura into killing Maya Amano with the Spear of Destiny, fulfilling the Oracle of Maia to destroy the world and causing Xibalba to be the only part of the world alive. Philemon averted this by wiping out the memories of the party, but Tatsuya Suou's memories remained and thus allowing Nyarlathotep to possibly revive the old timeline and cause world destruction.
Persona 2: Eternal Punishment
Nyarlathotep spreads new rumors in order to destroy the world via people's thoughts, such as manipulating the creation of the New World Order when they unearthed a mummy of Gozen from the construction site of Honmaru Park, and having them spread the rumor that via the mummy's kotodama they have the power to move the entire country. The rumor eventually leads to a seemingly nonsensical apocalyptic event where Kegare will be leaked to the center of the earth, which enrages the earth dragons and cause them to destroy the world while the New World Order escapes via Ameno Torifune to utopia. In reality, Gozen's story is likely fabricated in order to taunt Tatsuya Suou about the Other Side. The fabricated history also results in the creation of Ancestor Personas, which are all based on personalities created from the fabricated history of Gozen.
Further manipulation of the rumors results in the revival of Takahisa Kandori, the antagonist from Megami Ibunroku Persona, as well as Chizuru Ishigami advancing the power of the Joker Curse by first spreading that people can become a Joker themselves by using JOKER's services, then exaggerating it further by spreading the rumor where people can randomly become Joker without even contacting JOKER. The true purpose of the events is to collect Kegare from people infected by the Joker Curse, then send them all to the Earth's core and destroy it by enraging the earth dragons.
However, the plan is thwarted when the team defeats Gozen and sends the Ameno Torifune into the Collective Unconsciousness, resulting in the party climbing Monado Mandala to confront Nyarlathotep. They manage to defeat and banish him from the Persona universe, allowing Philemon to win the bet and eradicate the power of rumors once and for all.