Shadow of the Storm

Details
To save some time you can buy black priest robes from Thessalia's Fine Clothes in Varrock, black wizard hat from Betty's Emporium in Port Sarim, and dyed desert robes from black mushroom ink.

Walkthrough
South of Al Kaharid Bank talk to Fr. Reen. After you start, go to Uzer past the Shantay Pass south of Al-Kharid  (You can take a magic carpet there for 75 - 200 coins depending on whether or not you have the ring of charos and whether or not you've played the Rogue Trader game before.) After arriving in Uzer, talk to Father Badden (Be sure to talk to him about all your options). Afterwards, enter the dungeon.

When you try to enter the demon's lair, Evil Dave won't let you by because you aren't evil enough. You need to wear at least three items of black clothing. There are a number of ways to achieve this, including:


 * Dyeing a desert shirt and a desert robe black by picking a black mushroom outside of the dungeon and grinding it into black mushroom ink.
 * Wearing any three Black armour items.
 * Wearing Preist robes along with a Black wizard's hat

You also need to dye your Silverlight black using black mushroom ink. (Does NOT count as an 'evil' item)

Equip your three black items then tell Evil Dave you want to join the group and he will let you through the portal. Evil Dave will take you to the leader of the group, Denath. He will tell you that he needs your help to summon Agrith-Naar, since Josef left. He will tell you the incantation to use, which is different for everyone so you need to write it down.

Talk to Jennifer to get a demonic sigil mould. Go to Al-Kharid and make a demonic sigil with your silver bar. You can use this to chant the incantation with the group. You no longer need your black equipment to enter the portal.

Go back to the demon's lair and talk to Matthew. He will tell you to find the book that Josef dropped in order to defeat the demon. Talk to the Golem He will tell you that Denath killed Joseph, but not before Joseph hid the book in one of the kilns. [You must talk to golem before you can search the kilns] In Uzer, search all of the kilns (don't forget the one north of the magic carpet). You'll find a book. Read it, then return through the portal and discuss the content with Matthew. Strangely enough, the incantation for summoning the demon that's written in the book is backwards to the one that Denath told you to say...

will call you over to lead the incantation. Using your sigil, chant it in the order that he told you. (If you have forgotten it is in the book but backwards..) Suddenly, Denath will walk into the centre of the circle and go into another dimension. Denath was Agrith-Naar all along, and you actually desummoned him by saying the incantation backwards. This particular demon is much more powerful in his own dimension, so you need to get more people back in the circle and resummon him to defeat him!

Pick up the sigil that Denath dropped. Evil Dave, Eric, and Tanya will have run away, so chase after them through the portal. Tanya will get killed by ghosts, so take her sigil. Eric will be killed by falling stone. Tell Evil Dave that he needs to return to the lair. He will give you Eric's sigil then return to the lair. Exit the dungeon. Father Badden and Father Reen will both be there, so give them each a sigil and tell them to go to the demon's lair. Talk to the Clay Golem. He was programmed to never help summon demons, so go back into the dungeon, take the strange implement, use it on the golem, and take out the scroll about demons. Talk to the golem again, who will agree to help you.

Back in the demon's lair, go into the circle and talk to Matthew to get everyone to stand in the circle. Then chant the incantation with your sigil in the order that you read in the book. Agrith-Naar will appear, very angry that you summoned him. He will kill Matthew, then attack you. He is level 100. By default, he uses melee up close and casts fire blast at a distance, but if you use prayer, he'll automatically use the attack that you don't protect from. You must hit the final blow with Silverlight.

Once the demon is dead, try to unequip Silverlight. It will have been merged with the demon's blood.

Reward

 * 1 quest point
 * 10000 experience in any combat skill except prayer.
 * Silverlight turns into the Darklight sword - it has a special attack and is even more powerful against demons. It costs a bit more to replace it.