The Fourth Warrior, Task for the Ardent Cowl: Spirit Gem

Juna will send you to find the Ardent Mages in the catacombs under Mistmoore. They can be found on the ledge near the Myr'Dal, south west from the Ascent in the middle. The Head Ardent Mage will tell you to repower the cowl, you must do a favor for each of her children and herself. This is a task for one of the children. They want a Myr'Dal stone. By the Ardent mages there is a platform of Myr'dal with some highlightable chests near one of the named's thrones. To unlock them you need to kill any Myr'Dal until you get an update on your screen saying "You have found a Tarnished key." open the chest (could be random which chest it is) and get the stone. After this return to the Child to complete it. After completing all 4 quests talk to the head mage again to imbue your silk.