Some arcana cards that have weapons on them. Each weapon corresponds to one of the classes, for example, Blade Crafter and soldiers. When you fuse these cards with the correct class, you can permanently upgrade that unit's weapon, which will give them increased attack power and a higher critical chance. However the fusion is not always successful. Depending on how many cards you use, the success rate varies from 50% to 100%. You can check the rate before you perform the fusion. The more cards used, the higher the success rate.