Infinity Blade/WMG

The God-King is a body jumper
In addition to sucking all the power out of the warrior with the Infinity Blade, he also changes into the warrior's body, then goes and sires the next warrior himself. He is kind enough to leave all the gear with the mother, then goes back to his castle and waits for around 20 years.

Jossed by the novella and sequel

The God-King turns the dead warriors into the Death Knights
This is how the Death Knight can keep coming back.

Jossed by the novella and sequel as it's revealed that the God-King would just throw the dead warriors outside, letting commoners take his gear and give it back to the warriors' families.

The God-King raises the warriors in labs
The God-King has a whole bunch of guys he creates in test-tubes. The best bloodline gets to be the warrior, usually derived from the DNA of the previous warrior. The other ones get turned into the other enemies. Before the next warrior has come of age, the God-King abuses him, slaps him around, and tells him about how his father died. Finally, when the warrior is mature enough, the God-King sets him loose on top of a cliff.

Jossed by the novella and sequel

The world of Infinity Blade is post-apocalyptic Earth
It's already been revealed that the Deathless merely get their supposed immortality and strength from technology rather than magic like is thought all over their kingdom. What's to stop them from thinking that the Deathless are the societal elite from pre-apocalypse Earth that managed to survive, then created the technology to make them functionally immortal? Plus, there's the fact that the fortress in Infinity Blade II is built by a structure that looks a great deal like a large present-day dam...