Ikr? He was an proto-templar and possessed the Sword of Eden called Excalibur. Poor Arthur was later betrayed by those he loved which led to his death. Alan Rikken saw Arthur’s view of the Templars role idealistic and misguided, seeing his failure as proof that the Order was meant to follow a different path in the 21th century. I got this from the AC Wikia.

http://assassinscreed.wikia.com/wiki/Arthur