Pendragon or King Arthur Pendragon
Pendragon, also known as King Arthur Pendragon, is a role-playing game that is part of Chaosium's Basic Role-Playing game system. In the game players take the role of knights performing chivalric deeds in the tradition of the Arthurian legends. The game was originally published by Chaosium and later published by Green Knight Publishing and White Wolf, Inc.
From back of the 4th edition:
Live the glory of King Arthur's court. SMite bloodthirsty giants, crush treacherous invaders, brave the mysterious lands of faerie, and dabble in Celtic magic.
To become a knight of the Round Table you must uphold chivalric ideals of courage, honesty, fair play, and justice. Armed and armored, you are the law of trhe land, in a life or death struggle to join the fellowship of the Round Table.
Or, you might instead become a powerful magician using PENDROGON's new Celtic Magic System. Scour legendary Britain and the lands of faerie for sites of power, where you will work your fearsome spells and summonings.