You receive this quest automatically, after you have progressed to act three of the game, i.e. after you have completed The Hermit of Hadret House main quest. After you leave the city, you travel to Aedelwan's Bridge (M17,1). Head East, across the Woodend Plains to the Stormwall Gorge. Take the valley access that used to be inaccessible (M28,2).
Right after you descend, you will be halted by the members of the Leaden Key. You can try to talk your way out of the fight using high persuasion, however it's going to be tricky - start off by intimidating your opponents and then convince them that you'll let them live if they walk away. If you fail the conversation then you will be forced to take part in a rather difficult battle. After the fights start, attack mages and rogues, in the first place, and fighters afterwards. You can now go to the passage to the Elmshore (M35). Cross this location (talk to at Thaos the bridge) and take the South-Eastern exit. This way, you get to Hearthsong, i.e. the first are of the Twin Elms .
In Hearthsong talk to the Warrior at the palisade (M36,1) and then, to the Concerned Tribesmen a bit ahead. You can now go to The Passage of the Six building (M36,4). There, talk to Anamenfath Bethwl who is the head of the Anamenfath Council.
Exit the building and take the Eastern passage to the Elms' Reach. Head towards Twin Elms in the North-Eastern part of the reach (M37,6). At the bridge, listen to the conversation with Thaos and meet with delemgan sisters, i.e. Sidha and Rihenwn. You receive two new main quests.