Megaquake is an enemy only spell that appears in Fire Emblem Gaiden. It is a very obscure attack and seems to either be a scripted event or a spell, if it is a spell it does not appear in any unit's list of spells. It is used by Duma and Jamil. The spell itself damages all units on the map. In practice this spell is very rarely used and it is possible for the player to complete the game without ever seeing it.
Although Megaquake technically does not have a magic type, it is most similar to Black Magic and has thus been categorized with them.