Demon Slayer

Official quest description: A mighty demon is being summoned to destroy the city of Varrock. You find out you are the one destined to stop him (or at least try).
Release date: Unknown
Start: Talk to The Gypsy located in a tent around Varrock's centre.
Difficulty: *****
Length: Medium
Members only: No
Requirements:
Capable of defeating a level 27 demon who is aided by 1 level 20 Dark Wizard & 2 level 7 Dark Wizards.
Items needed:
1 coin
25 bones (un-noted)
1 Bucket of water or an empty bucket
Recommended Items
Armour that has a high Magic defence
Food
Monsters to kill:
Delrith (Level 27)
Rewards
3 Quest Points
Silverlight
Spoiler warning: Plot and/or ending details follow.
edit Walkthrough
After being paid one coin, the gypsy will predict your future. She sees a vision of you fighting a mighty demon named Delrith, who was summoned during the Fifth Age and subsequently defeated by the hero Wally. She tells you that a cult of dark wizards is attempting to summon Delrith, and she sends you to talk to Sir Prysin for help. The gypsy will also give you an incantation. Write this down, as it is different for each player. Five different words make up the incantation and are randomly ordered. Because some are repeated, there are more than 125 possible permutations. If you do forget the incantation, simply talk to the gypsy again at any time during the quest, and she'll remind you of it.
Go north to Varrock Castle and find Sir Prysin on the western side of the castle. Tell him that the gypsy sent you. Ask for Silverlight, the weapon needed to kill Delrith. He will say he has kept it hidden because of its power and needs three keys to open the case it is stored in.
First key
In the northwest corner of the castle is a set of stairs. Climb them twice to reach a room with Captain Rovin. Speak to him and tell him that a demon is going to invade the city, that you have to defeat it, and you need his Silverlight key. Select the option that says "Sir Prysin said you would give me the key" and then select the option that says "Then why did he give you the key?" After bringing up Sir Prysin's clumsiness he will hand it to you.
Second key
The key is stuck down a drain behind the kitchen in the castle. Go get a bucket just upstairs the kitchen and fill it with water at any water source; the sink in the kitchen is close and convenient. Use the bucket of water on the drain and the key will be washed into the sewers. Enter the Varrock sewers using the manhole located immediately east of the castle's keep. Follow the northwestern path until you get to a stream and search around there. It is just beside the skeletons under a pile of rubble. Search for a rusty key on the ground; it will not appear on the Third key
Gather 25 sets of regular, unnoted bones. You can buy them on the Grand Exchange or kill almost any monster and take its bones. Head to the Wizards' Tower, located south of Draynor Village. On the second floor of the tower, talk to Wizard Traiborn about the key, and he will ask for 25 bones. Give them to him and he will perform a ritual, then he will give you the third key.
Banishing Delrith

Delrith
Return to Varrock Castle and give the three keys to Sir Prysin. He will give you Silverlight.
Now that you have Silverlight, it's time to fight Delrith. Head to the bank and withdraw any food, potions and armour you might wish to bring to the battle. Armour with good magic defence, such as leather armour, is good to bring. Make sure you're wielding Silverlight and head to the stone circle immediately southeast of Varrock's south entrance, where the dark wizards are.
A short cutscene will play in which three dark wizards and Denath summon Delrith, a tall, red and black demon. Denath will then notice you and run off.
This is a single-combat area so, when the cutscene ends, run to and attack Delrith. He is combat level 27, which can be intimidating to lower-leveled players, but he is extremely weak for his combat level because he has not had enough time since his summoning to gain full access to his powers. He has 7 hitpoints and can hit a maximum of 3 damage, so most players should not have much trouble fighting him. By contrast, the level 20 dark wizard that fights alongside Delrith can hit up to 6 damage and both the level 20 and level 7 wizards can reduce your stats. Wearing armour that defends against magic will help against their spells. It is a good idea to gather lots of tuna. If you are a high-enough level, choose the strength option and you may be able to defeat Delrith in one hit.
Once you defeat Delrith, he will collapse onto the ground, and a prompt will appear. Choose the incantation that the gypsy gave you at the start of the quest and Delrith will be banished from this dimension. If you choose the wrong incantation, Delrith will be revived and the fight will resume until you defeat him again.