Unlocking the quest
You can unlock this quest during one the trips to [Tri-Stone]. The first time you can go there is right after initiating the main quest [The Fire of the Mountain]. Find Thane in the northern area of the location, and select the upper right dialogue option, which is Changing fate. Muria will ask you to deliver her three alchemy ingredients - Stalker Bone, Mordent Drew and Carven Stone.
Find Karn and talk to him
Karn can be found at the entrance to [The Cauldron]; it's not possible to miss him since he's an integral part of the main quest [The Fire of the Mountain]. Talk to the Ancient just after defeating Constructs, and select Your shaman's craft. Karn will then you that you can obtain the Stalker Bone in The Cauldron.
[TIP: Make sure to select the right dialogue option, because Karn is also a quest giver on his own; he has the [Lost and Found] side quest.]
Obtain the Stalker Bone
You'll receive the Stalker Bone automatically in the course of completing the main quest [The Fire of the Mountain]; namely during the exploration of [The Cauldron], and even more precisely - after the mandatory battle with a Stalker (the above screen). You should receive the ingredient afterwards.
[TIP: Tips on how to defeat the Stalker are in the description of [The Fire of the Mountain] quest.]
Return to Karn and ask him about the Mordent Drew
It's best to return to Karn only after exploring the whole of [The Cauldron] and interacting with the last mechanism (after defeating Gharn). Leave the dungeon and find the Ancient near the exit of [The Cauldron]. Trigger a conversation and select Mordent Dew, thanks to which you'll learn that the last alchemy ingredient can be obtained from Stingers, which inhabit, among other places, the Drenchfort.
Get the Mordent Drew off a Stinger
Mordent Drew is obtained practically automatically, namely during an encounter with Stingers (simply let them attack you). It can take place even before reaching [Drenchfort] (the above screen - a battle in the Shadow Gorge), or while exploring it.
Go back to Karn and ask about the Craven Stone
It's best to start looking for Karn only after completing the quest [The Tears of the Mountain], which is after leaving Drenchfort. You should look for him in the location of another main quest [To Move a Mountain], namely by the entrance to [The Lost Temple]. Talk to the Ancient by selecting Craven Stone in the dialogue. He'll tell you that the stone can be obtained from one of the Construct Sentinels inside the Temple.
Get the Craven Stone off a Construct Sentinel
You'll obtain the Craven Stone automatically after defeating the first Construct Sentinel, which is right at the beginning of [The Lost Temple] during the [To Move a Mountain] quest. You can't miss it anyway, because taking the Sentinel (and other Constructs) down is necessary to unlock access to further parts of the temple.
Report back to Muria with the ingredients
Fast travel to [Tri-Stone] and find Muria, which should be at the same as before, which is in the southern part of the location. Tell her you've retrieved the materials. The main reward for completing this quest is a talisman - the Grim Amulet. Apart from that, you'll also receive 329 XP and 1500 gold.
[TIP: The Grim Amulet is quite a unique item, because it's only property is accumulating the Reaper Energy. If you enjoy using the Reaper Form, it pays off to equip it.]