The Imprisoning War (Or Seal War) was mentioned for the first time in The Legend of Zelda: A Link to the Past.
This legend tells of Ganondorf the Thief's accidental entry into the Sacred Realm, followed immediately by him laying claim to the Triforce, and becoming Ganon, the Demon King, which further resulted in transforming the Sacred Realm into the Dark World. The legend then goes on to relate Ganon's invasion of Hyrule (whether it was his minions, his evil power, or Ganon himself that invaded is still up for debate), to which the King of Hyrule responded by calling upon the Knights of Hyrule and the seven Sages. It is said the Knights perished during the invasion so as to give time to the Sages to cast a seal on the entrance to the Dark World, sealing Ganon within.
The first published English account of the Imprisoning War was not a direct translation from the Japanese, but a new variation dreamed up by Nintendo of America. Fans wanting to learn the real tale have been forced to find a true, canonical translation online, such as those found here