Battle with the Lord of Lightning!
From the Super Mario Wiki
Before the mission, Mario and Cappy's time with controls are interrupted by a large thud. They go outside to confront Bowser stood on the Ruined Dragon's head. The dragon strikes the Odyssey with a magenta lightning bolt, wrecking it. The Odyssey crash lands in the Ruined Kingdom, which is reduced to rubble. It is required to be repaired with three Power Moons. Some swords embedded in the ground may be removed by Cappy leading up a few phases of stairs. One last sword reveals a spark pylon leading to the Tower of Crumbleden where the battle begins. The Ruined Dragon flies up to the scene and prepares to attack. Mario must avoid his breath of lightning. Then the Ruined Dragon will hammer his head into the ground and becomes lightning free, the lightning spreads across the floor leaving Mario to jump over or evade it. Then he may attack by jumping onto the dragon's head, removing the swords which breaks a crown with Bowser's face on it and Ground Pounding a glowing part of his head. Repeating twice more will make the Ruined Dragon faint and rest on the tower, giving Mario and Cappy victory and a Multi Moon.
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