Thunderbolt

Thunderbolt is a greatsword. It inflicts lightning-based damage onto targets and is thus more effective to enemies with a weakness to lightning-based attacks.

Source
Thunderbolt can be purchased from arms vendors in Altissia, Cartanica, Tenebrae, and Gralea for 5,000 gil. It can also be found as a collectable in the Malmalam Thicket and the Pitioss Ruins. It can be sold back to shops for 2,500 gil.