Map of Sinsear
1 - Voitve of Leaf
2 - Votive of Flame
3 - Votive of Water
4 - The Crying Eyes
5 - Key
In order to get to the key, you have to obtain three other items, guarded by groups of beasts.
Votive of Leaf can be found at the copse of Bog Thresha.
Votive of Flame is in possession of Sprite Flame Champion.
Votive of Water is guarded by Banshen Priestess.
After obtaining all items, go to so called The Crying Eyes. These are three statues. A water will flow from their eyes, if you stand on the plate in front of them. Make each statue to "cry" and then use the nearby levers in same order (left, right, middle).
When you're done, the gate will open and you'll be able to get inside to get the key.
Bring the artifact to Biserane in House of Sorrows and you'll discover the terrible truth. Fae will betray you and as it turns out, he is a servant of Gadflow.
Once the conversation is over, you'll be attacked by a group of Tuatha, led by mercenary Myrcyr. Kill quickly his helpers and then wait until he attacks Taibreah. He won't pay attention on you then, so you will be able to attack him.
Once he is dead, you'll get another quest.