Diara is a level 3 White Magic spell which inflicts moderate damage against all undead enemies. It has no effect on other types of enemies. In the NES and Origins releases, it inflicts between 40 and 160 damage, while in subsequent releases it inflicts damage depending on the caster's Intelligence.
The spell can be bought at Elfheim and can be learned by the White Mage and White Wizard job classes. In the GBA/PSP/iOS releases it costs 8 MP to cast.