The spell originally appeared as the final tier of the Thunder-school of Black Magic in the Nintendo Entertainment System version of Final Fantasy, but was eventually replaced with the traditional Thundaga (サンダガ, Sandaga?) spell in future re-releases of the game. While sharing the name of two recurring spells in later English localizations, it is considered a separate ability in the Japanese versions of the respective titles the spell appears in, serving as homages to the original Famicom versions of Final Fantasy.
Thundaga, known as LIT3 in the NES version, is a level 6 Black Magic spell, which inflicts heavy lightning damage to all enemies. In the original NES release, it inflicted between 60 and 240 lightning damage. Thundaga may be cast by the Dark Wizard and Chaos.
In the original Japanese version, this spell was named "Thunger" (サンガー, Sangā?). The spell's name was replaced with the modern "Thundaga" (サンダガ, Sandaga?) starting from the WonderSwan Color release. However, the Japanese version of the Pixel Remaster reverts the name to Thundger, due to the release being heavily based on the original Famicom release.
Thundaja is a spell exclusive to the Esper Adrammelech during his boss fight. It causes heavy Thunder damage and inflicts the Stop status. This ability is similar to the Ark Blast Quickening concurrence.
In the Japanese version, this spell was named Thunger (サンガー, Sangā?) rather than Thundaja (サンダジャ, Sandaja?) to pay the original Final Fantasy homage in which the third tier of Lightning-elemental spell was named so. It was likely renamed to Thundaja in the localization for consistency with the other Esper abilities that used the -Ja suffix.
Venerable strength!Garland using Thundaga
Garland has Thundaga as a Bravery attack. It calls a lightning bolt down around the opponent, and comes in both ground and aerial variants. Thundaga costs 30 CP to equip and 100 AP to master.