These order of knights were founded by Tiber Septim. The player joins this faction at the highest ranking, Champion of Cyrodiil when they complete the main quest (theirs actually only that one ranking in-game). The player is the only one who can join this faction. Their are no benefits to joining this faction other than the fancy title. Their have been 6 other champions in Tamriel's history with the player being the 7th.