Radiant Breath (レディアントブレス, Redianto Buresu?) is a recurring ability in the series. The attack has mostly been associated with Rydia's Mist Dragon eidolon starting from the 3D remake of Final Fantasy IV, serving as a Holy-elemental replacement for the summon's original Mist Breath attack from the 2D versions of the game.
While being known as Radiant Breath in newer English localizations of the 2D versions Final Fantasy IV, the Mist Dragon's attack is still the Mist Breath attack in original Japanese language version. This is due to newer localizations borrowing terms from the 3D versions of the game. 
Radiant Breath is the Mist Dragon's summon attack in the 3D versions of the game. It deals Holy-elemental damage to all targets and its summoning cost is 35 MP.
Radiant Breath is Mist Dragon's summoned attack. It has a casting time of 3 and costs 20 MP to summon, the damage is based on Rydia's current HP.
Radiant Breath is Mist Dragon's summoned attack, costing 35 MP and deals holy-elemental damage to all enemies. Its spell power is 120.
Radiant Breath is a Blue Mage spell learned from Wyverns. It deals light damage to enemies within a fan-shaped area of effect originating from the caster with additional Slow and Silence effects. It costs 4 Blue Magic Points to set. When set, it grants HP +5 and CHR +1.
Radiant Breath is also an enemy ability used by the Wyvern family.
Radiant Breath is an enemy ability used by the Hydra boss during the "A Relic Reborn" quest. When the ability is used, it will cause the Hydra to spray out a breath of radiant crystals to the targets and will inflict the Infirmity status effect for a period of 20 seconds.
Radiant Breath is the ability used by Rydia during battle.