This description is ironic because it paints Satan as a king sitting on his throne wearing a crown. This is what Satan wanted but in this position he is king of the underworld. He no longer has the power he had in heaven, and this certainly does not compare with God's throne. In Heaven he was one of God's highest angels, but he became greedy and threw it all away. It is almost too painful to think of Satan in this situation. He had just about everything in heaven but he wanted more. We have to pity his pride because of all that it has lost him. His diadem and scepter are pointed high towards heaven, yet he falls lower in Hell. This description uses metaphor in an ironic way. Diadem and scepter are two things associated with royal power and authority. Satan possesses both of these yet he has no power. This could be another example of how Milton views the monarchy in England. The king  has his scepter and diadem, but he has lost much of his power.