The forces of Harad have made their way to South Ithilien and Osgiliath, including Kethat-sai, a massive and deadly Mûmak.
'The Haradrim are bringing powerful beasts known as Mûmakil up from the south to create the foundation of the Morgul-host. These tusked creatures are nearly impervious to our swords and arrows and Kethat-sai seems to be the largest and strongest of them. If you were able to somehow stop this beast from making it across the Anduin, we would be most indebted to you.'
Kethat-sai can be found somewhere in South Ithilien or Osgiliath.
You should defeat Kethat-sai.