With the key entrusted to him by Zero's dying father, Holy Dragon Castle transforms into Holy Dragon Grand God, the pinnacle of the Thunder System and the ultimate technique of the Thunder Sword. The giant cannon on his chest, the Thunder God Cannon, amplifies and fires the lightning energy of Holy Dragon Machine. It can receive the power of the will of the knights who are in tune with Zero, and can be equipped as a Union Sword, allowing it to exert even greater power by coordinating with other units, including Holy Dragon Machine.

It gathers the light of the Union Six Machine Soldiers into his fist, creates the Union Sword, a sword of light, and defeats the Storm God.