The Elder Scrolls IV: Oblivion Guide
Knights of the Nine
Knights of the Nine is a downloadable questline for the PC and Xbox 360 versions of The Elder Scrolls IV: Oblivion. For those playing on the PS3 console, Knights of the Nine is included on the standalone Oblivion disc, so there's no need to purchase or download anything.
If you are playing Oblivion on the Xbox 360 console and have access to Xbox Live, you can purchase and download Knights of the Nine from the Xbox Live Marketplace for 800 MP at the time of this writing. Unfortunately, if you do not have access to Xbox Live and only own the standalone version of the game, you won't be able to download this extra questline. However, The Knights of the Nine questline is included in the Oblivion Game of the Year edition, which also contains the Shivering Isle expansion pack.
If you are playing the PC version of Oblivion, you can purchase the Knights of the Nine from Bethesda's online store at http://obliviondownloads.com
Alternatively for PC users, Knights of the Nine is included in the 'The Elder Scrolls IV: Knights of the Nine Oblivion Downloadable Content Collection,' which is available in stores.