For those who don't know.
In order to access the new content, one much have
1. Saved Dusk of Oolacile from the golden crystal golem behind the hydra in Darkroot Basin
2. Acquired the Lordvessel from Gywnevere
You now must enter The Duke's Archives and take the elevator up. There will be a crystal golem in the right side of the balcony up the stairs. Killing him will reward you with The Broken Pendant.
Now backtrack all the way back to Darkroot Basin, to where you originally saved Dusk. There will be a black orb. Touch it.
There is no need to prepare for this trip, as you can warp in and out whenever you wish due to the Lordvessel's ability.
Enjoy, and do not succumb to the Abyss like Artorias before you.