N card is dog food, which is used to nourish the mind. Because all the N cards come out with 2 stars, they can be directly used to help Great God rise to 3 stars. When they are trained to level 20, they can rise to 3 stars by eating N cards as the material for the gods to rise to 4 stars, and so on. Of course, if you think too many N cards can be used to upgrade the Great God, you can just eat a bunch of quick upgrades.