The PCs are all elves, very angry elves. They are part of a very selective group of warriors who answer to the White Council in Kyonin, the largest elven kingdom in the world of Golarion. Someone discovered that the River Kingdoms was once an ancient elven empire that fell to the changes of time. For thousands of years, monsters, humans, and just about everything else turned the region upside down. Most of the River Kingdoms are somewhat civilized, while a small sliver of land called the Stolen Lands, remains untamed and savage.
The Kingdom of Brevoy has offered hardy adventurers a chance to clear the Stolen Lands of all hostiles and pacify the area into a charter under the rulership of Brevoy should any party of adventurers succeed. Because the White Council believes this land should be under elven control, that is where the all-elf party comes in.
The goal of the mission is simple: prevent the Stolen Lands from becoming another hellspawn of human taint. How this is done is up to the PCs. There is no time limit, as there is much to do and lots to face off against. The human incursion is not the only threat, as lots and lots of foul and evil things lurk in the land. Once you are in, you are on your own.
Good luck, and Hail Kyonin!