"This magic infused greatsword was forged by the Hyrulean royal family using lighting from the Hyrule Hills. Attack when the blade glows golden to expel lighting."
Hyrule Compendium

The Great Thunderblade is an item from The Legend of Zelda: Breath of the Wild. It is a two-handed elemental sword filled with thunder power, shocking and causing electric damage to enemies.

One can be obtained by defeating a Stalnox that can be found southwest Ishto Soh Shrine and respawns after every Blood Moon.

See also