When Artorias wait for his end after being defeated by the Chosen Undead The Stranger presents him a deal, a quest into the lands os Westeros to stop the coming of the long night which will bring the world to an Age of Dark.

The fire fades, summer ends, and the long night approaches. Perhaps it is already here. Voices and screams. Flickers of memory. A task undertaken, and a promise unfulfilled. Even now, only embers remain. And so it is. That ash seeketh embers. (Fusion AU)

Artorias ends up in Westeros after his defeat at the hands of the Chosen Undead. How will the events of ASOIAF change with Artorias the Abysswalker in it?

The nature of man is a cyclical dilemma of endless fighting of will and desire. After the fall of countless civilizations filled with magic and wonder that faded into history, humans live an illusionary life that is peace. But nothing remains the same in the natural order, and progress demands a lord to mantle the weight of souls of the world. To bear the curse of Undeath.