As you may already know, Artorias had made a mysterious covenant with the creatures of the dark and it cursed his sword. So why don't we talk a little bit about it. The covenant is mysterious. No one knows why Artorias made this covenant in the first place but we do know that it gave him the power to traverse the abyss. But also it had cursed his greatsword. Artorias apparently needed his sword to be cursed so that he could kill the ghosts that inhabited New Londo. But because his sword didn't have a curse put on it, he was not able to even hurt the ghosts.
Artorias cursed his sword and he was then truly able to lay waste to the ghosts. Without problem, he killed every ghosts that came his way. The cursed version of his sword also had special properties that allowed Artorias to do physical damage to an enemy. But before he cursed his sword, it had divine properties that dealt magic damage and holy damage at a significant rate. When the creatures of the dark try to avoid Artorias, they usually get killed before they can even make it out alive. Mostly because he is unmatched with a greatsword. While Artorias was fighting the forces of the abyss, he had to destroy the Four Kings, who were tempted by the power of LifeDrain.
When they were banished, Artorias continued onto Oolacile to free the citizens and save Princess Dusk from the corruption of Manus. Artorias used everything he had to save his wolf companion Sif and hopelessly became corrupted by the dark. His already cursed sword became corrupted as well and he left himself become a servant of Manus. Oolacile's liberation became hopeless, to the citizens of Oolacile there was no hope for them now with Artorias corrupted by darkness. They had only one chance left, and they called upon The Chosen Undead to save Oolacile and stop the spread of the abyss.