The other Promise that we can't Keep ( Fire Emblem X Kingdom Hearts)

In Fódlan there is a boy named Y/n who does not remember who he was but he carried something in him. Y/n holds the Keyblade between light and dark. As Y/n meets new friends he said was then going to Garreg Mach to learn. For months he witnessed the events and learn that friendship everyone has is falling apart. Y/n learns what he have to do. He needed to find someway while he had to face friends that he made a promised.