Daniel 5:20 (ESV)

20 But when his heart was lifted up and his spirit was hardened so that he dealt proudly, he was brought down from his kingly throne, and his glory was taken from him.

Belshazzar knows the story of what happened to Nebuchadnezzar when he cried out, “This is the great Babylon that I have built.” It is the Lord who then showed his power. He caused the mighty Nebuchadnezzar to become insane and thus removed him from his throne.