Knight Artorias was once a proud knight, renowned the world over for his heroic deeds done for the kingdom of Lordran. The people loved him, and he loved the people he defended.

His seemingly everlasting glory would end abruptly, however, when the neighboring kingdom of Oolacile called for his aid when Manus, lord of the abyss had begun to destroy Oolacile. Knight Artorias rushed to their aid, bringing along his battle-trained grey wolf, Sif, to help him fight Manus. They fought effortlessly through Manus' minions, having battled much more powerful foes in the past, and got to Manus swiftly, expecting this to be another quick fight.

He was very wrong.

Manus quickly overtook Artorias with the powers of the Abyss, and simply putting the wolf to sleep, seeing no threat from it. Thus, the great knight Artorias became the horror we know today:

Artorias the Abyss walker.

This is a crossover with Dark Souls. I don't own anything aside from the story itself, although I'm sure that was painfully obvious.