By River and Root is the last quest in the Murkmire Storyline. You can start this storyline by speaking with Famia Mercius in Lilmoth, the first available city in Murkmire.
There are a total of 7 story quests to complete in the zone:
Sunken Treasure, Missing in Murkmire, Whispers in the Wood, Death and Dreaming, The Swamp and the Serpent, The Remnant of Argon and By River and Root.
This achievement is one of the prerequisites of "Hero of Murkmire" as well.