Through Death's Gate

 is a main quest in.

Synopsis
The Acolyte's vision showed the Watcher mysterious ruins somewhere outside of Dyrford. The Leaden Key has sent operatives to find the site, though their purpose is unknown. It's up to the Watcher to make it known.

Walkthrough
You automatically get this quest after completing Never Far from the Queen.

Go to the Temple of Berath in Dyrford Village and convince Beodmar to tell you where Clîaban Rilag is. Travel to Dyrford Crossing and thence north to Clîaban Rilag. A Glanfathan scouting party is blocking a bridge leading to the main entrance of the ruins. Either fight them to gain access to the bridge, or cross the stream and use the back entrance. (The artifact that leads you to the conclusion of Fragments of a Scattered Faith is towards the back entrance.)

Level 1. Enter the ruins and fight off a couple of Trolls. To the north (left) lies mostly some monsters; to the south (right) you'll also find a side room with some extra treasure. Clear out both branches now to save yourself some trouble later. Both roads lead to the same junction, where you'll find some enemy druids and be able to spot your eventual goal across an uncrossable chasm.

Proceed further south until you reach a natural cave with a number of spore creatures. This room is also full of traps, so be careful charging across the floor while still in combat mode. The stairs down are on the far side of the cavern.

Level 2. Cross the main hallway. The central door ahead of you has a level 8 lock, pick it for xp or search the key: go north (left) and fight your way to the northernmost (top-left) part of the map. Open a door to find a room with a pool and a green statue (and some Phantoms). In the pool is the Engwithan key. Proceed directly south from the pool room to find an unlocked door that will also take you to the central chamber.

If you take the southern hallway, it leads to a room bathed in red light with a dais that contains an Adra Animat and his companions. There you can pick up The Core necklace and the contents of two armor racks.

The central chamber contains a beautiful adra statue and some Rain Blights. Cross it and take the eastern door (top-right). You will need the Engwithan key to open this door. Then go up the stairs to Level 1 again. Approach the Engwithan Machine and click on the purple glowing spirit to complete the quest.

On your way out, notice the shimmering blue portal off to the side of the room with the Machine. Enter it for a scripted scene. At the appropriate moment, stop in the river tunnel and grab onto the edge; then send someone with Constitution 16 or Iverra's Diving Helmet to pick up the shiny thing. It is The Rose of Salthollow, a unique morning star. The encounter drops you in a natural cave on the north side of Level 1, from which (assuming you cleared the enemies out of here earlier) you can easily exit the ruins.

Note that if you did not kill the Glanfathans standing guard before, and you choose to exit through the "front" door now, you will have to fight them.