Notre Sagesse, the "land of philosophers", is an island nation located in the southern sea between Hoshido and Nohr. It is the home of the Rainbow Sage, who resides within the Sevenfold Sanctuary atop Mount Sagesse. Throughout history, many have come to Notre Sagesse to gain the Rainbow Sage's blessing of great power, though only four have survived the ordeal: the first was King Sumeragi, the second King Garon, the third a little-known old knight, and the fourth Prince Xander. Notre Sagesse also houses some diseases native to the islands that can be fatal if not treated quickly.
During the Nohr-Hoshido conflict, Notre Sagesse remained neutral, though King Garon planned to conquer it eventually.
After learning from Leo that Xander had traveled to Notre Sagesse and gained great power from the Rainbow Sage there, Leo gives Corrin a warp tome strong enough to teleport them to Notre Sagesse and back. In a town below Mount Sagesse, they meet an old man who directs them to the Sevenfold Sanctuary, but warns that few who seek out the Sage survive meeting him as they must fight through the Sanctuary's seven floors first. Corrin resolves to attempt the test despite the danger.
Upon defeating the last of the Sevenfold Sanctuary's illusory warriors, Corrin enters the final door, only to return to the town at the bottom of the mountain where the old man waits. He reveals himself as the Rainbow Sage, explaining that the divine power he "gave" those who passed his test was not something he could bestow, but merely the added strength gained from surviving the Sanctuary's gauntlet. He reveals to them the secret power of the sword Yato. He asks Takumi for his Fujin Yumi and performs the Seal of Flames ritual, handing back Corrin the Noble Yato and Takumi his bow. As a parting gift, he hands Corrin a hand-drawn map with directions to an underground tunnel that leads directly to the Nohrian capital of Windmire from the Demon's Falls.
King Garon orders Corrin to conquer Notre Sagesse for Nohr and destroy the Hoshidan army reportedly stationed there. Xander suggests that they should seek out the Rainbow Sage to obtain his gift of divine power, revealing that he and Garon have both done so already. Corrin's party travels to Notre Sagesse via the Nohrian port of Dia, and are surprised to find no sign of a Hoshidan force. They learn from a local resident that the Hoshidans, under Hinoka's command, have forcibly placed the Rainbow Sage under their protection and occupy the Sevenfold Sanctuary.
The Nohrian forces climb Mount Sagesse and defeats the Hoshidans, managing to avoid killing or seriously injuring any of them as they ordered. Corrin meets the Rainbow Sage, who explains that they have already gained the "divine power" by weathering the battle through the Sanctuary and becoming stronger. The Sage also reveals the secret power of Yato. Since he realizes that the Nohrian hero isn't with them, the Sage merely awakens a seed that was already sown and tells them to merge with Leo's Brynhildr to perform the Seal of Flames ritual, and it can transform into the Grim Yato.
As Corrin prepares to depart, however, Iago magically projects himself to convey King Garon's latest order: Corrin must kill the Rainbow Sage to deny his power to Hoshido. They protest, as Ryoma had already gained the Sage's power, but Iago warns that defying an order from the king would be treason. The Sage protects them by allowing himself to die, insisting that it was his time; as an ancient dragon he was prone to succumb to madness and inflict great destruction on the world should he linger further. With Notre Sagesse secure under Nohrian occupation, Corrin's party returns to Nohr.
Sometime after Corrin's army leaves Notre Sagesse, Elise contracts an illness from the island and ends up getting sick.
In Notre Sagesse, Corrin's army talks to an old man for information on where to find the Rainbow Sage. The old man recognizes Corrin’s sword as being the Yato and asks them if they seek power. After Corrin states that they want an answer to their question, the old man reveals that the Rainbow Sage is at the top of the Sevenfold Sanctuary at the top of Mount Sagesse.
The group climbs the mountain and reaches the tower. Battle commences once they are inside, and they all fight their way to the top. Once the last enemy is defeated, they go through the door at the end of the room. Surprisingly, they are back in the room that where they had talked to the old man.
The old man reveals himself to be the Rainbow Sage. Corrin states that they were told to meet a dragon that would help end the war. The Rainbow makes the Yato transform into the Alpha Yato, and mentions that in order to make the Seal of Flames complete, they would need the other four divine weapons. With those, the Seal of Flames would become the Fire Emblem.
The Rainbow Sage collapses onto the ground, and though Corrin tells Sakura and Elise to heal him, the Rainbow Sage states that human magic will not work on him. He tells them that he is the dragon that they had been searching for. The Rainbow Sage tells of the war from long past between twelve dragons seeking to control the world. He was one of them, and created the Yato and the other divine weapons, and with those, humans joined in the battle.
The Sage tells Corrin that he had given the Yato the last of his power, and asks them to use the Yato to make their own destiny. The Sage dies, and Corrin states that he will not let his sacrifice be in vain. Corrin tells the group that they will go to the Bottomless Canyon.
Known people from Notre Sagesse
Notre Sagesse literally means Our Wisdom in French.