Demon Slayer

Demon Slayer is a quest in which you need to defeat a demon to prevent it from destroying Varrock.

Start
Talk to the Gypsy in her tent in the center of Varrock, near the clothing shop. When you speak to her she will try to get you to pay her to read your future, so give her 1 coin. Next, ask her about Delrith and she will tell you a story.

After finishing her story, she sends you to talk to Sir Prysin about the sword required to kill Delrith.

Sir Prysin's Silverlight
Sir Prysin can be found in a small room on the west hand side in Varrock castle. Talk to him and he will seem surprised when you mention Delrith. As it turns out, Wally, the hero from the gypsy's tale, is the great-grandfather of the knight. He has Silverlight in his possession, but it's inside a locked box. The box requires 3 keys to open, and you need to locate the 3 keys.

The 1st key
Go to the room in the northwest corner of the castle and climb the stairs. You will be inside a room with guards. Climb the ladder in this room. Captain Rovin will be walking around. Talk to him and tell him that what you're talking about is important. Tell him that a demon is going to invade the city, and that you must defeat it. He will hand over his key.

The 2nd key


Go to the downstairs kitchen in the northeast part of the castle. The key is inside the drain, but you are unable to reach it. Use the bucket of water in your inventory on the drain, so the key falls into the sewer.

Go down the manhole that is east of the castle, indicated by a red exclamation point. Travel up the main passage of the sewer (the one on the left) and make your way down until you find a pile of corpses and the second key, as a red spot on the minimap.

The 3rd key
The last key requries you to travel to the Wizard's Tower and seek Wizard Traiborn. The Wizard's Tower is south of Draynor Village. You will find Wizard Traiborn in the second story of the tower.

When you ask him for the key, he says he'll need 25 bones to perform a ritual. You could kill the wizard around the tower to collect up the bones, or go to a nearby chicken coop and kill some chickens. Once you have given the wizard all 25 bones, he will unlock the box and give you the final key.

Obtaining Silverlight
Return to Sir Prysin and give him the three keys and he will unlock the box and give you the sword Silverlight.

The correct incantation
Go back to the Gypsy and ask her what the correct incantation is to banish Delrith. Make sure you write it down!

Killing the demon
Get your equipment ready for combat. You will need food, armour, and Silverlight. Head to the stone circle, south of Varrock's southern entrance. As you approach the stone circle, you will see a cut scene with three dark wizards praying to the stone in the center. They begin to chant and then from the stone circle the demon rises.

Ensure you have Silverlight equipped. Enter the circle, and kill any wizards that attack you. Approach the Demon and attack it. When he is in his weak state, say the correct incantation and the demon will be banished.