The Seal of the Sages is a key element to the story of The Legend of Zelda: A Link to the Past. It is a seal that was placed on the Dark World to seal inside the evil coming from that world to the Light World.
According to A Link to the Past, the King of Hyrule commanded the Seven Sages to place the seal on the Sacred Realm, which had been tainted by evil and become the Dark World. This seal imprisoned Ganon, the source of the evil, within the Dark World. Many years later in a campaign known as the War of the Wizard, Ganon attempted to break the seal by transporting the descendants of the sages who had created the seal to the Dark World. To accomplish this, Ganon embodied a wizard named Agahnim so he could have access to the Light World.
In the manga version, Agahnim was intrigued by the power of the Triforce, and researched on breaking the seal to acquire the relic's mighty power. To accomplish this, he kidnapped the Seven Maidens and took the Book of Mudora from Link in order to sacrifice them through a ritual, which he ultimately succeeded, thus opening the portal between the Light and Dark Worlds.