Start by talking to Father Reen outside of the Al Khaird bank. He wants you to help the dark mages summon Agrith Naar, a powerful demon, then kill it once and for all. He asks you to head to Uzer and meet up with Father Badden.
Head through the Shantay Pass, and either walk or ride a carpet to Uzer. Talk to Father Badden to find out more details, and fill him in on what's happened in Uzer. Enter the ruins and pick a black mushroom. Grind it on the pestle and mortar, and use the ink on Silverlight. If you need to dye your desert robes, repeat with making the ink and using on them.
Wearing the three black items, plus the dyed Silverlight, talk to Evil Dave outside of the portal and ask to join their group. He will then Escort you though the throne room.
During the conversation with Denath, he will tell you an incantation. WRITE IT DOWN! It differs for every person.
Talk to Jennifer to get the Demonic sigil mould. Now talk to Matthew and ask what happened to Josef. Now go out and talk to the golem. He will tell you that Josef was killed by Denath. Search all of the kilns (there is also one north of the magic carpet) to find the demonic tome.
Read through the book. Notice that the incantation in the book is opposite of what Denath told you. Now leave and go to a furnace, and with your silver bar, make the sigil.
Return to Uzer. Grab the strange implement in the ruins before heading into the throne room and talking to Matthew. Discuss the book with him, then after the conversation, wait until Denath calls for the ritual to start.
Take your place in the circle then lead the incantation. After the ritual, Matthew will explain what happened. Take the sigil lying on the ground, then exit through the portal. Tanya is killed by the ghosts. Pick up her sigil too.
Talk to Evil Dave. Ask him what happened. Tell him to get back to the throne room, and he will give you Eric's sigil. Reutrn upstairs to the desert to see a sandstorm.
Killing Agrith Naar
Talk to the golem. He will tell you he cannot help. You then suggest to reprogram him again. With the strange implement, use it on the golem and you will remove the instructions inhibiting him. Talk to the golem again and he will go into the portal.
Now talk to both Fathers and give them the sigils. They will head inside. Return back to the throne room and talk to Matthew to get started. Take your place, then lead the incantation. Remember, it's backwards as of what you did the first time!
He uses magic and melee. If you use protection prayers, he will only use what you aren't praying against. You MUST deal the final blow with Silverlight.
Afterwards, his blood will stain Silverlight, making it Darklight, a more powerful version of the sword. You can then choose to earn experience in a combat skill of choice (excluding prayer and summoning).