Start off by heading to the House of Ballads M2(9) and speaking with Alyn Shir. She can be found at the Hall of Accolades.
With that done, speak to Glianal, sitting beside. You will learn a bit about the Codex of Destiny, however sadly Fae won't be eager to tell you too much.
After speaking with Alyn again, you will be sent to the tree known as Nyralim. It can be found in the southern part of The Sidhe M8(17).
Caer Nyralim map
1 - Lymarra
2 - Nyralim
After reaching Caer Nyralim M8(17), keep heading onwards. The tree can be found at the very end of the path.
The ancient creature will tell you to kill the Troll leader, Gnarsh.
In order to find her, head to Haxhi Dam in Haxhi M7(12). You will have to battle a Rock Troll before being able to enter it.
Kill him quickly and head inside, where you will come across Alyn Shir. After a short conversation she will join you and help in finding Gnarsh.
Haxhi Dam map
1 - Alyn Shir
2 - Gnarsh
3 - Exit
Keep heading onwards and apart from Trolls you will also come across Tuatha groups. Try eliminating mages and archers in the first place and attack the warriors only afterwards.
Gnarsh is waiting for you at the very end of the tunnel. Her attacks are almost identical to the ones used by other Trolls, but deal far more damage. The best way of avoiding getting hit is dealing 2-3 blows and quickly jumping backwards. Gnarsh is capable of leading out combos, so dodge further than usual. Also remember about entering Reckoning mode by the end of the battle to receive more experience points than usual.
With the fight over, speak to Alyn Shir and afterwards return to Nyralim.
The tree will thank you for completing the mission and send you to the city of Ysa M8(9). Apart from that, if you choose the middle option and succeed in persuasion, you will receive an amulet which boosts your elemental resistance by 10%. After reaching the destination point, the door will open and you will be able to enter the city of the immortal.