There is a ring event to unlock the entrance to this instance. Without the ring event first being cleared it is impossible to enter. Details are as followed:
Slay the two cyclopes guarding the entrance to the Seer Stone area at ( -16, -48, -405 ) /waypoint -16, -48, -405. This will initiate the ring event.
Slay every cyclopes in the area around the Seer Stone.
After a few moments, slay the Sandfury apostles and protectors that spawn immediately around the Seer Stone at ( 44, -57, -412 ) /waypoint 44, -57, -412. A message in white text will flash across you screen telling you that the door to Ancient's Table has been unlocked.
Click on the knocker to zone-in. You may have to zoom in the camera to have the knocker render on the entrance door.
There may be slight variations in the spawning of the cyclopes in the area around the Seer Stone. Essentially, the most important thing is to kill the two cyclopes in step one as this will reset the ring event regardless of how many cyclopes in the center area have already been killed.