Farron Keep is a large swamp, and often blends into the Road of Sacrifices for many adventurers. Home to the Ghru, there are many treasures to be found within this swamp, as well as the ill-fated Abyss Watchers.
This zone has a covenant leader in, the Old Wolf of Farron. He can be found up the very tall ladder to the right-hand side of the Keep Ruins bonfire.
If you are embered, you may be invaded by the Watchdogs of Farron, whose goal it is to stop you from reaching the keep. They take the form of reddish blue (not purple) phantoms, of which 2 can be active at a time. Because 2 connection slots are reserved for these phantoms, the number of friendly phantoms that can be summoned is limited. You can only have one friendly phantom in this area unless you use Dried Fingers