Takk sends you to the Hidden Vale, which is an instance of The Shattered Vale. The entrance is at -2340, 9, -70 in Antonica. You'll want a group, but note that this zone is restricted to players level 80 and over.
Kill six Darkened Spirits (80^^^) for your update. They float around outside of the perimeter of the druid ring.
The clue you're searching for is the horned altar in the middle of the druid ring . You'll have to clear the zone to get to it. Right click the altar for your update.
Takk has explained to me a vision that he had recently. He has seen dark figures surrounding an altar, drawing in spiritual energy and consuming it. He has asked that I investigate this ominous dream, and see if there is a way to put a stop to it. He believes that the area in his vision was an area known as the Hidden Vale.
I must investigate the Hidden Vale, as instructed by the elder sage, Takk.
I must beware of the dark shadows I was forewarned about by Takk.
I have destroyed the dark spirits I found there.
I must search for any other clues within the Vale.
I found a dark altar, but I was not able to destroy it.
I have finished investigating the Vale.
I must return to Takk within Graystone Yard.
I have investigated the Hidden Vale, and must return to Takk within Graystone Yard.
I have spoken with Takk in Graystone Yard.
I have completed the task Takk gave to me.
I was able to find the dark spirits possessing the altar within the Hidden Vale. Although I was able to destroy the apparitions, the altar continues to draw in more energy within it. I must find a way to destroy or purify this altar.