Dark Souls Downer: Play The Whole Game to Get the DLC
According to Eurogamer, you might have to replay the entire Dark Souls to access the Artorias of the Abyss DLC.
Defeat Hydra at Darkroot Basin
Defeat Crystal Golem at Darkroot Basin and save Dusk of Oolacile
Defeat Crystal Golem at Duke’s Archive and get Broken Pendant
Touch Black Fog at lake in Darkroot Basin where the player defeated Hydra
If you don’t do what’s listed above, there’s no other way to access the DLC.
It could take around 20 hours of gameplay to reach the Duke’s Archives – depending on your style of play and knowledge of the game. But also, once you enter New Game+ mode, it becomes more difficult to face enemies, who are tougher in this mode.