In order to reach the key, you must get past the Facetteleon.
Felicitations stranger, let me now properly introduce myself! I am Dyria D'Arquelyght, Archon of the Dryads. Forgive me for not revealing myself earlier; I needed to get to back to the safety of my sanctuary as quickly as possible. This abduction was no coincidence. The Inquisitors wanted to force me to reveal the location of the key to them, and but for you they would have succeeded!
I owe you my life, and am grateful for your deeds. Your name shall never be forgotten amongst my kindred.
So - you are looking for the key as well? Alas, I have bad tidings for you. The key rests in the safest possible place, the Crystal Cave to the north, which is guarded by a fearsome monster, the Facetteleon. I fear that your quest is futile. No one has ever succeeded in overcoming this guardian.