Not much is known of the legitimacy of Arthur Artorias II’s life. At one point, he was a knight, not known to many but praised for his ferociousness in battle by friends and family. He hid a dark secret, however, that being his lineage as an Abyssborn. He had been given up by his parents and instead grew up an orphan before deciding to go out on his own.
To him, it began when he returned to his hut in the woods, for he lived alone and with no family, and witnessed a giant of a man roaming through the trees.
The mere sight of Makios alone would have been enough, but Arthur was struck with a vision; Terrible pain and agony, Makios standing atop the world, death, and destruction following in his wake. The vision drove him mad. Makios took pity on him, however, and recruited Arthur into his own army and declared him a Knight of Makios. He later was acquainted with Olivia, Makios' liege, whom he fell needlessly in love with.
Arthur is insane to the highest degree, driven by the vision he had seen, and lives, hoping only to be useful to Makios to aid him in his mission. He craves violence, death, destruction, and excels in delivering it as a means of training. It is all he cares for.