The 'Artorias of the Abyss' expansion will add new explorable areas, bosses, enemies, non-playable characters and equipment, according to IGN.
It will be made available later this year on Xbox 360 and PS3 for 1200 Microsoft Points / £10 / $15.
Namco Bandai has also confirmed that Dark Souls will also be available through Steam, following a petition signed by over 20,000 fans objecting to it coming to Games for Windows.
Dark Souls sold well after its release on Xbox 360 and PS3 in October, with 1.5 million units shipped worldwide.
Dark Souls: Prepare to Die Edition, which will feature the full console game as well as the 'Artorias of the Abyss' expansion, will be available on PC from August 24.
> Read Digital Spy's Dark Souls review
Watch the PC announcement trailer for Dark Souls: Prepare to Die Edition below: