Demon's Home

The Story
Talk to Duke Horacio Of Lumbridge to hear the story of the death of his friend in the Wilderness. His death is the 14th so far not caused by anything in the Wilderness mainlands. Duke Horacio tell you that his friend died in the Demonic Ruins, in the very north of the Wilderness. He will tell you to go talk to the King Of Varrock, who is investigating this phenomenon.

Your Job
The King Of Varrock will inform you that you must have completed the Demon Slayer Quest and behold at least a Combat Level of 60 to win his trust. At this point, if you have not completed Demon Slayer, you should. Then bring your Silverlight and 20 Fire Runes with you to talk to the king. He will say that you are fit for the job, and that you should go immediately to the Demonic Ruins. Once you make it there, search for the hole in the ground. Enter there.

The City of Furthu-Kaz
With your Silverlight and your runes, enter the gates underground. You will get a message saying that you can not enter unless a spell is broken. Use your Fire Runes on the gate and enter the city. In the city, you will encounter many friendly demons. Many Lesser Demons, Greater Demons, Imps, and Black Demons (only for members) will roam the roads. Talk to King Frtauioulari in his castle. He will observe you and then detect the Silverlight. He will grow enraged and send you into the arenas.

The Ytup Colosseum
You will be released of your chains and put against 20 Imps, all Level-2. Once they are defeated, you will be put in a fight against 5 Lesser Demons, all at Level-82. Finally, you will fight 2 Greater Demons, both Level-92. After defeating all of them, the King will grow enraged. He will put you up against his most powerful Guard, Hulinbgtyure, at Level-120. After defeating him, the king will order you to his castle.

Back To The Castle
The King will allow you to speak. Be sure to ask him about the deaths. He will tell you the story. The Demonic Ruins was not just a fortificated city, but it was also the location of Furthu-Kaz. He will tell you that the city had to be moved underground due to deaths. He will say that Greater Demons guarded the old foundation from destruction, and at the death of one of them, they would put a curse instantly killing the killers. After this story is done, the king will tell you the secret of teleporting back. He will also allow you to wear his forsaken Demon Armor and allow you FULL ACCESS to the city. (To teleport, you need 45 magic, 5 Fire Runes, and a Death Rune.)

Extra Part: Duke's Rewards
Talk to Duke Horacio Of Lumbridge who will give you 50,000 coins and a Rune Platebody as a reward.

Trivia

 * If you kill a Greater Demon in the Demonic Ruins, strangely, the curse is not activated on you.