Gullinkambi is an item that first appears in Book IV of Fire Emblem Heroes.
It is a horn given to the Order of Heroes by Dream-King Freyr. When used at the Dream Gate, it can return someone to the real world so long as they can imagine and desire to return to it.
In Norse Mythology, Gullinkambi is a rooster that lives in Valhalla. It is said that it's one of the three roosters whose crowing marks the start of Ragnarok.