Ruins of the Lost Realm
To Sauron, now openly declared in Mordor, the West is an afterthought. He has already won there: the ring-smiths of Eregion are gone, Gil-galad is dead, Elendil is dead, their kingdoms are gone. There is no strength there to withstand him, so he instead musters his forces for Lothlorien, Erebor, Dale, Rohan, and Gondor.
But there is yet one thorn in his side: Rivendell and its lord Elrond. It was Elrond’s father who’s pleas before the Valar defeated the shadow long ago. Elrond’s friend and liege-lord lead the Last Alliance. And as Elrond had a part in the forging of the rings perhaps he may even know where the ruling ring now sits.
The Last Homely House is not so easily besieged as it was during the War of the Elves and Sauron. The lands have changed and the valley is now hidden even from the eye.
So Sauron has sent Black Númenóreans—descendants of the Dúnedain who fell under the Shadow as servants of Ar-Pharazôn or other lesser Southern realms—to scour the West and find Rivendell so that the armies of Umbar and orcs may remove this last nagging resistance in the West.
But also in the West is Gandalf, and the Grey wizard is one of the few who may yet prevent the fall of Rivendell. Even the Wise are not so sure of Sauron’s intent—could this all be a feint to draw them away from Mordor’s more direct aims?—so Gandalf must rely on those willing free people who may be able to find the Black Númenóreans, drive them back, and keep Rivendell a safe haven where the hopes of the world may yet be kindled.