Whatever thou art, stay away. Soon, I will be consumed...by them, by the Dark. Thou are strong, human. Surely thine kind are more than pure dark. I beg of thee, the spread of the abyss, must be stopped. Ah, Sif, there you are... [cough] All of you, forgive me. For I have availed you nothing.
{In my opinion this dialogue was intended for a different version of the fight that was scrapped by From Soft, a version where Artorias hasn't fully succumbed to the Abyss when the player meets him. The version we experience in the game gives us the consumed and corrupted shell of Artorias, I've said this in another post, but he comes at you like a wild beast, not the skilled swordsman that Artorias would have been. It's very clear in my mind. He has sacrificed his greatshield to protect Sif and retreated from the Chasm, without question with the intention of returning once he has recovered. He ends up at the arena, and realizes he cannot overcome the corruption of the Abyss, possibly choosing this as a final resting place before the Abyss takes him. That is when the Chosen Undead shows up. . "Whatever thou art, stay away!" As the Chosen Undead approaches, Artorias warns him to stay away. He does not want to hurt this human, but the Abyss is drawn towards humanity, and Manus still seeks his precious pendant. The Chosen Undead is also unrelenting in his journey and continues his advance. Cutscene before the beginning of the boss fight. 100% hp. "Soon... I will be consumed... by them, by the Dark" He informs the Chosen Undead that he has been touched by the Abyss and he will fall to it's corruption soon enough. He is losing control over himself. 75% hp. "REEEUUUUGHHHHHHHH!" Presumably Artorias' mind is still somewhat intact but he is increasingly losing control of his actions. The wild screams are representative of the pain he's going through, fighting a losing battle within himself to maintain control of his body. I imagine during some of the screams he would stop fighting you and do things like hold his head in agony, turn away to divert his attention elsewhere, and walk around randomly. This is Artorias gaining small footholds of control over the Abyss, flares of the unbendable will of steel that he was renowned for. This would replace the Super Saiyen power up thing he does in the ingame version and would probably stop happening after the next quote. 100-50% hp. "Thou art strong human, surely thine kind are more than pure Dark" His mind slips further, Artorias gives way in his resistance to the fight against the Chosen Undead. Possibly hallucinating it as some sort of duel, he expresses admiration of the human's strength in battle. 50% hp. "I beg of thee, the spread of the Abyss... must be stopped" A desperate request as he realizes/accepts he will most likely lose both battles. Artorias still wants to see his mission completed. 25% hp. "Ah, Sif! There you are! All of you... forgive me... for I have availed you nothing" A final moment of clarity from a broken mind. He hallucinates his friends and comrades, and apologizes to them for his utter failure. This, along with the previous quote, demonstrates an enormous sense of duty on Artorias' part. I imagine this being in a cutscene after the player finishes his health bar. Maybe from Artorias' pov, or maybe from the Chosen Undeads pov just watching Artorias turn away and talk to people who aren't there before he finally dies. . Purely some more speculation on my part, but I think the fact that From decided to go with a voice actor who sounds somewhat young and boyish could mean that Artorias may have been fairly young himself. We know Ornstein and Gough were around during the war against the dragons, as they both have Dragon Slayer titles, but Artorias is never mentioned when it comes to the dragons. I don't think he was actually around back then. He distinguishes himself by hunting Darkraiths and walking the Abyss, he seems to just be Gwyns resident expert when it comes to the scary dark things, but this wasn't an issue during the war against the dragons. The four knights of Gwyn, he grabs two distinguished veterans of the war and two young knights who have shown potential, I feel this is an institution that would be put in place after Gwyns rise to power, a way of maintaining order and demonstrating strength in his rule. Same goes for the Lord's Blades and Ciaran, you don't need assassins in a war against massive beasts, you need them after you have won that war. To help maintain power and control. I don't know how to format properly on Reddit yet, this entire post looks messy to me, sorry :( Edited for eyes and fleshed out some thoughts for clarity.}