Knight Artorias, also known as the 'Abysswalker', was one of the four knights of Gwyn.
There is a possibility that he was present during the fall of New Londo as he traveled to New Londo when the Four Kings fell to the dark and hunted the Darkwraiths. To do this, he made a covenant with the beasts of the Abyss (possibly Kaathe or Frampt) that allowed him to traverse it.
When Oolacile fell to the dark, he was awarded the Silver Pendant for facing the Abyss:
One of the ancient treasures of Anor Londo.
Presented to Artorias for facing the Abyss.
Effectively deflects the Dark of the Abyss, especially in its magic forms.
He traveled to Oolacile but was defeated by Manus. Artorias sacrificed himself to protect his wolf companion Sif, using his greatshield to raise a barrier around the pup. He fled from Manus, but his exposure to the abyss overcame him and he fell to its corruption. By the time the the Chosen Undead arrives, it is too late. Artorias is by then completely overtaken by the corruption of the Abyss and attacks the player.
His grave, marked with his greatsword, is placed at the Oolacile Sanctuary area (later known as the Darkroot Garden) and is guarded by Sif, Alvina's clan of Forest Hunters, and possibly also Lord's Blade Ciaran.