If you need to obtain some Mourner's Gear, you have to kill a level 11 Mourner along Arandar Pass. Be Careful! Their first attack will drain all of your combat stats to 20.
After killing them and picking their gear up, the shirt will be bloody and the trousers will need repaired. Head to Taverley look for the Druid washing clothes in the lake. Steal some soap from the nearby basket, and, with a bucket of water in your inventory, clean the shirt.
For the trousers, bring two silk and two bear fur to the dye seller in Lletya. Give her the silk and furs to repair the trousers.
After starting the quest, you can kill level 108 Mourner's under the Mourner's HQ in West Ardougne, and they will drop a set that doesn't need washed or repaired.
Start by speaking to Arianwyn. If you don't have your crystal trinket, ask for a replacement first. Arianwyn will catch you up to speed on what's going on: Thorgel is now working with us, and something is happening in the temple.
Arianwyn would like to join you in investigating what's going on. He will tell you he needs a set of mourner gear for him and yourself. He will then ask for you to create a distraction. And finally, he needs a way of getting there.
He will hand you a blank teleport crystal, saying you need to chant to it in the area where you will teleport in at.
Preparation to Enter
Go to West Ardougne, and enter the Mourner HQ. Climb down the ladder into the basement. Kill one of the level 108 mourners down there and pick up their gear for Arianwyn if you don't already have a second set.
In the southeast room, search the desk for a new and tarnished key. Talk to the head mourner and talk to him about the gnome. Talk to him about releasing the gnome, then go into the cell with the gnome and talk to him about the quest.
Now enter the mines through the door to the west. Continue through towards the temple of light, and in the clearing right before the dead slaves and the entrance to the temple, enchant the crystal.
To the Altar!
Return to Arianwyn in Lletya. Give him the mourner set and the crystal seed, and explain the distraction. When you're ready to go, he will teleport both of you to the location where you enchanted the crystal.
Arianwyn will teleport you first, then follow after. Lead him to through the Temple. Climb up the steps at the entrance, head south and climb up the other set of stairs, then north down the steps into the center.
Note: If your cannot go through the cyan door, then talk to Arianwyn and ask him to leave you. Then, go through to the dwarven camp area of the Underground Pass and enter through the back way to the Death Altar, rotate the mirror, then return to Arianwyn in Lletya.
Once through the Cyan door with Arianwyn, rotate the mirror and go into the room with the death altar and talk to Thorgel, then Arianwyn. Investigate the energized pillar through the light doors.
Talk to Arianwyn again, and then use the crystal trinket on the pillar. The screen will then flash. Speak to Arianwyn about this, then talk to him again and ask for a death talisman.
Enter the Death Altar and speak to Thorgel. Have him help you up on the altar.
Quickly run out of the death altar, through the light doors, rotate the mirror, go through the cyan door, climb up the north stairs, head to the east, touch the large crystal, then return to the energized pillar.
If you don't reach it in time, you will have to return to the death altar. If you make it in time, you will be teleported to a strange place. Search the dead slave to get a hatchet and knife. You may need to cut and fix a crystal tree.
The puzzle is different for everyone. You can rotate some crystal cores to redirect the light beams. The first step is to get the blue beam to the yellow receiver on the other side of the room.
After cancelling out the yellow beam, a red beam will start. You now have to combine the red and blue beams to make a magenta beam. Direct the magenta beam into the green receiver. A green beam will now start.
Now, combine the blue and green beams to make a cyan beam. Direct it to the red receiver. Afterwards, combine all three beams to make it turn into a white beam. Direct that into the colourless receiver.
A ladder will now appear. Climb up it and you will now be on platforms. Crystals will pop up and down on them in varying patterns. Time your movements to avoid them. Make your way all the way around to the crystal ballista and ry to shoot the moving target.
After shooting it, the screen will shake. Return back to the beginning and climb down the ladder. The large stone door will now be open. Go through to find a dead elf messenger. Search the body to find a report.
Read the report then go back through the door and exit through the portal. Go back into the death altar and talk to Thorgel. He will tell you that Arianwyn went back to Lletya. If you want to free the gnome (completely optional), do so now.
Return to Arianwyn when ready, and explain to him what happened.