The Oracle of Maia is a poem and prophecy written by Tatsuya Sudou.
True Nature
Nyarlathotep dictated the poem to Sudou, who conversely passed it to Akinari Kashihara, so he could add it to the supposedly mystical tome he was writing, In Lak'ech. When the time came, he used the Joker's escapades along with those of the Masked Circle to ensure the Oracle's prophecies came to pass, ensuring reality was weakened enough in the area of Sumaru City so the release of In Lak'ech would inevitably allow the creation/return of the Fuhrer and his Last Battalion.
This supplied Nyarlathotep with a powerful, organized military force he hoped would reach Mount Katatsumuri and reach the core of Sumaru's powers. While the bulk of the Battalion's forces was destroyed before reaching the location, Nyarlathotep himself, disguised as the Fuhrer, did manage to reach said core and force a confrontation between himself and Philemon's champions. While he lost, he had previously influenced the deranged teacher Maya Okamura to kill Maya Amano with the Spear of Destiny, fulfilling one of the last lines of the poem and causing the Grand Cross to occur, halting the Earth's rotation and subsequently leading to its destruction from the violent inertia of all on the surface of the planet.
Only Sumaru City remained untouched by the destruction of the planet, being the only vestige of humanity left floating above the now-dead Earth.
However, Philemon appeared to the group of friends and gave them the possibility to negate the timeline by ensuring they had never met. All but Tatsuya Suou agreed, creating the Eternal Punishment timeline and killing the Innocent Sin one, fulfilling the last line of the poem. However, the dead timeline remained a threat due to Suou's refusal to surrender his memories of his friends, and became a major plot point known as the Other Side.
In Eternal Punishment, due to In Lak'ech never being written, the Oracle failed to come true, although it was seen written on the walls of Sudou's room in the Sanitarium.
Quotes
The twinkling star of Pleiades sets time in motion;
Dance of pleasure, Feast of shadow, Tongue of stranger, Atonement lights above.
The lion's roar echoes throughout...
The 5 skulls shine in Hades...
The holy cross shines in the Heavens...
The moment the stars stop rising...
The beat of the virgin Maia also ceases...
Paradise remains on Earth...
-- The unfinished Oracle in Persona 2: Eternal Punishment