The War of Heroes (英雄戦争 Eiyū Sensō) is the in-universe term used to describe the events of Fire Emblem: Mystery of the Emblem Book 2 and Fire Emblem: New Mystery of the Emblem. These events take place after the War of Shadows depicted in Fire Emblem: Shadow Dragon and the Blade of Light, Fire Emblem: Mystery of the Emblem Book 1, and Fire Emblem: Shadow Dragon.
After Medeus is killed, concluding the War of Shadows and bringing peace to Archanea, Hardin becomes Emperor of Archanea by marrying his beloved Nyna, while Marth returns to Altea with Caeda. Some time later, Hardin discovers that Nyna was never truly in love with him.
The evil wizard Gharnef uses this to his advantage, posing as a simple merchant to trick Hardin into possession of the Darksphere, which slowly corrupts his mind. Hardin, under the Darksphere's influence, leads a campaign to subjugate and rule over the entire world. Marth takes up arms against him, and after a long campaign, Hardin is defeated, and the Binding Shield is restored.
After the war, Marth returns to Altea and marries Caeda. He also becomes the ruler of the continent of Archanea.