Below is the legend as told by Queen Serenity to her Royal Court during the final days of the Silver Millenium.

There are six among the people of Earth, six whom have a hidden strength.  These six are the Millenium Knights.  Back before the beginning of the Silver Millenium, before the Imperium Silver Crystal, the evil of Bentaris threatened to take this solar system once again.  To stop this threat Sailor Galaxia and six curageous Knights rose against Bentaris and even with their powers were not enough to emerge victorious.

They then retreated and through an epic journey with a young peasant girl learned what they needed to create the Imperium Silver Crystal.  They made a final assault on Bentaris's fortress on the sun and with their combined power and that of the Imperium Silver Crystal drove Bentaris off, nearly destroying him.  With Bentaris defeated the Prince of the Moon married the peasant girl and so began the Silver Millenium.

Combining her own might with that of the Millenium Knights Sailor Galaxia spread the power of the Senshi throughout the galaxy, beginning the era of the Senshi.  The Millenium Knight's though would perish in the war against Chaos, yet their power and courage lives on.  Every generation six individuals on the planet Earth are born with their power, when they die the power is passed on.

This might will remain hidden until the day they are needed once again.  The day Bentaris, or an evil much greater returns to threaten the universe...a day I pray never comes to pass.

Ok that's all you guys are getting.  The full story of that battle will be written, I'm working on the outline now and should be completed in a few years.  Until then I hope you kinda understand what the Millenium Knights are now.