This is a ridiculous theory which links the Halo Universe with the Infinity Blade universe.
Infinity Blade is set in the time of ancient humans. Magic is advanced technology from the Forerunners. Because Siris is a deathless and reincarnation of Ausar the Terrible, he is constantly reborn until the age of Spartans. John is a reincarnation of Siris, and Ausar, and that's where he gets his immense fighting skills.
[Edited on 12.28.2012 1:49 PM PST]