After the Great Invocation fails, go up to the pedestal where Y'Berion has collapsed and talk to Cor Angar. Learn everything you can from Cor Angar and he will eventually give you this quest. There was a party sent on ahead of your character to the Orc Cemetery, the location that may have been shown to you in the vision, but there has been no contact since; you are to go and contact them to report back. Talas will be your guide again, so go down to the temple courtyard and talk with Talas about going to the Orc Cemetery. Talas will lead you back up the hill again, towards the place where he was mugged in Almanac, but detour at the last second and head for a second bridge across the waters to your sharp left. He won't accompany any further.
Across the bridge you will encounter a group of Biters and, beyond them, some Orc Hunters and a fallen Templar. You'll find many fallen Templars and, were the route inside the cemetery not so linear, I would recommend you follow the corpses to the end. Head towards the caves and use the winch to open the wooden gate and head into the Orc cemetery (read: catacombs). You will find a lot of Orc Scouts in the cemetery, as well as potion-ridden mummies in the alcoves. Loot the corpses of the fallen Templars and follow the route around a corner to find Baal Lukor fighting a number of Orcs.
In the first burial chamber you entered, there are four barred alcoves. The one on your left is unlocked with a switch in the alcove directly above it. Another alcove on the left wall has a switch, this unlocks one of the barred alcoves on the right wall. This unlocked alcove contains a switch inside for another locked alcove, and that one likewise contains a switch inside for the last locked one.
Help Baal Lukor fight his Orcs and learn that he is the last of the first party that came to the cemetery. He will follow YOU around now and suggest you examine the three connecting chambers to the one you are currently in. The forward-right chamber descends to a room where there are a number of Orcs waiting for you to fight. Head into the darkened shallow cave in that area and find the switch on the wall on the left wall. This lifts the grating and, beyond that, you will discover half of a torn parchment. Return to the original chamber you found Baal Lukor and search the forward-right chamber to discover lots of skeletons and the other half of the torn parchment. Back in the original chamber, Baal Lukor will come to a realization that the runes drawn on the walls of this chamber can help him decipher the completed parchment - it is a teleportation scroll that must be used in a specific place.
Now follow the left-most path (the fancy one) and you will come to a chamber where an Orc Warrior stands guard in the center. The Orc Warrior is probably going to be a tough fight regardless of how you handle him. Off to the wings of the room are Orc Scouts. Once clear, head all the way into the room and Baal Lukor will claim that this chamber looks muhc like the one in the vision, then head over to a closed-off tunnel that was originally on the right side of the room and claim that this must be the place where the teleportation scrolls is to be used. Correction: this is the place where you will use the teleportation scroll.
On the other side of the door, use the winch to lower to door, and get Baal Lukor to start following you again. The left tunnel contains herbs for healing. The path ahead of you leads to a dark tomb where a skeleton is impaled on an altar with a sword. Baal Lukor will apparently go mad and attack you. You don't need to kill him or even beat him up if you don't want to. Once you're done here, head back out of the cemetery and go back down to the Sect Camp to report to Cor Angar. You've found no Sleeper and everything rests on Y'Berion waking up.