Despite the similar name in English releases, the spell is not the same as the recurring Diaga ability that originated in the original Final Fantasy, which is known as Dadia (ダディア, Dadia?, lit. Dadia) in Japanese.
Diaga is an Enfeebling Magic spell associated with White Magic. It can be used by Red Mages from level 12 and White Mages from level 18. An upgrade of the spell Dia, it lowers enemies' Defense within the affected area and also deals gradual Light-elemental damage. Uses 12 MP to cast, it takes 1.5 seconds to cast, and 6 for subsequent casting.
In addition, stronger versions called Diaga II and Diaga III appear as enemy abilities.
Diaga is an ability tied to the Lohengrin weapon, serving as the highest form of holy-elemental magic available to players. When used, the ability will inflict three holy-elemental attacks to all enemies caught in range.
Dia means "day" in Portuguese, Spanish and Occitan.
In ancient Greek religion and folklore, Dia means "heavenly", "divine", or "she who belongs to Zeus". The name is attributed to several figures and treated like an honorific or title.