What is happening in the dale of udun as frodo and sam approach it?

Crouched under a great boulder they sat facing back westward and did not speak for some time. Then Frodo breathed a sigh of relief. 'It's passed,' he said. They stood up, and then they both stared in wonder. Away to their left, southward, against a sky that was turning grey, the peaks and high ridges of the great range began to appear dark and black, visible shapes. Light was growing behind them. Slowly it crept towards the North. There was battle far above in the high spaces of the air. The billowing clouds of Mordor were being driven back, their edges tattering as a wind out of the living world came up and swept the fumes and smokes towards the dark land of their home. Under the lifting skirts of the dreary canopy dim light leaked into Mordor like pale morning through the grimed window of a prison.

'Look at it, Mr. Frodo!' said Sam. 'Look at it! The wind's changed. Something's happening. He's not having it all his own way. His darkness is breaking up out in the world there. I wish I could see what is going on!'

It was the morning of the fifteenth of March, and over the Vale of Anduin the Sun was rising above the eastern shadow, and the south-west wind was blowing. Théoden lay dying on the Pelennor Fields.

As Frodo and Sam stood and gazed, the rim of light spread all along the line of the Ephel Dúath, and then they saw a shape, moving at a great speed out of the West, at first only a black speck against the glimmering strip above the mountain-tops, but growing, until it plunged like a bolt into the dark canopy and passed high above them. As it went it sent out a long shrill cry, the voice of a Nazgûl; but this cry no longer held any terror for them: it was a cry of woe and dismay, ill tidings for the Dark Tower. The Lord of the Ring-wraiths had met his doom.