Spoiler Alert
After you beat the god king, and buy the infinity blade and put it in the slot in the cave, you have to beat 3 deathless (Level 50, 100, and 150. Then a huge door opens and you go into a chamber filled with god kings in tubes. Then you fight the level 200 ZERO MECH!
Groundhogs huias about
Infinity Blade, v1.4.1