This quest begins immediately after Counting Sheep. Now that you have all the shards, you can return to the Cave of Gold to unlock the temple and end the nightmares.
Enter the Cave of Gold and Defeat the Guards
When you enter the Cave of Gold, you’ll need to run through the cave system to reach a large room with some guards in it. When you reach a body of water along the way, dive in on the left side to swim under the rocks, then emerge on the other side where the brazier is. Beyond this you’ll find the room full of guards. When you kill them all, a cutscene will play during which Kassandra enters. You’ll need to kill some more enemies with her before continuing.
How to Solve the Ancient Light Mechanism to Open the Door
Once you’ve defeated all the enemies with Kassandra, you’ll need to solve the ancient light mechanism to open the door. This only requires a few steps, and you don’t need to use all the reflectors to complete it. To see how you can do it as quickly as possible, check the gif below.
Now that the door is open, you can proceed through the cave with Kassandra. You’ll encounter various items generated by the Isu artifact from Kassandra’s past, and eventually she’ll stop with one. At this point you’ll need to approach the artifact alone. When you do, a cutscene will begin and Kassandra will eventually intervene. At the end of the scene the quest will end, but your time with Kassandra isn’t over yet. Regardless of how you speak to her with the dialogue options, you’ll be meeting her for a drink at Kiltaraglen.