God Unit is a designation of various classes that are weak to specific weaponry designed for gods and god-like entities.
The classification of "God units" in the Fire Emblem Series makes its official appearance in Fire Emblem Echoes: Shadows of Valentia. However, many Fire Emblem games prior have major antagonists who are only specifically weak to weaponry designed for gods and god-like entities.
Here is a list of "God" units:
Here is a list of skills that are effective to "God" units: