At player level 28 the feature Infuse is unlocked. Here you can equip the force book cards you have earned throughout the game. There are 3 categories: attack, defense and support. Each category can hold 5 different cards.

Depending on the cards you use in a category you can unlock Infusion Combination. You can find information about these when clicking the cogwheel in the top left of the window. If all the cards in the combination reach a certain level you can unlock higher level Infusion Combination skills.

This guide is sourced from yabusame.

Depending on the cards you use in a category you can unlock Infusion Combination. You can find information about these when clicking the cogwheel in the top left of the window. If all the cards in the combination reach a certain level you can unlock higher level Infusion Combination skills.