Demon Slayer is a subquest of the Dimension of Disaster quest.
Speak to Gypsy Aris in New Varrock square. She tells you that you've got a future ahead of you, but you tell her that you already know that your future was to kill Delrith. She tells you that you need Silverlight to kill Delrith. You will recall from your first time killing Delrith that that Gideon Bede or Sir Prysin has the key to silverlight. Gypsy Aris will tell you that Gideon Bede is dead, but that Sir Prysin now lives in a house north-west of the church.
Finding the Key
Searching the drain.
Go to Sir Prysin's house, and ask him about the key you need. He says he had the key, but lost it. The key is in a random hiding spot inside, upstairs, or right outside the house in the drain to the south. (The key's location is the same for every replay.) Now, go to the church, and on the right on the entrance, there is a trapdoor. Go in the trapdoor. You see that the Silverlight is covered by a barrier, without the 3 spirit guardians. Click on the barrier, and then go back to Gypsy Aris, and tell her about this barrier. She will tell you that the guardian spirits have moved on, and you'll need to find them. She gives you a device. The ghosts are located in these locations, and you can visit them in any order you wish to. It is recommended that you go to the Spirit of the Mind ghost last, as it is really close to the church.
- The first ghost is in the Blue Moon Inn, possessing Dr Harlow.
Spirit of the Body
Go to Dr Harlow and tell him he's possessed. You'll need a potion to sober him up, so the ghost can leave his body. Go to the apothecary and ask if he can make a potion to sober someone up. He tells you that he can, but he needs a cabbage, a rotten tomato and a hair of a dog. He tells you that there is a dog in Gertrude's kennel.
- You can get the cabbage south of the Blue Moon Inn.
- You can get the rotten tomato north of the Blue Moon Inn.
- Gertrude lives to the south of the western bank, close to the tanner, where the Zaros Emissary usually is. Next to her house, you see a kennel. Do every option available except for pulling your hand out, do that last. You'll get a dog's hair when you finally remove your hand.
Now, go back to the Apothecary. He'll take your ingredients and make the potion. Go back to Dr Harlow. If you try to give the potion to him, he'll refuse it. Buy a glass of beer from the bartender. Do not drink the beer, but use the potion on the glass of beer. Then, give the glass to Dr Harlow. The ghost will get out of his body, and Harlow gets angry at you for sobering him up. Talk to the ghost. He'll tell you that he wants you to kill anything in the bar. Alternatively, you may also ask him to kill yourself instead (you will not lose anything). Talk to the ghost again. He's happy, and tells you that his barrier is lifted. You now see a short cutscene of his barrier disappearing.
Spirit of Faith
Now, go to the graveyard, north-west of Gypsy Aris' tent, and go to the north side, next to the statue of Zemouregal. You'll see that you can investigate a stack of tombs. Investigate it. One of the three ghosts will pop out. Talk to him. He tells you to simply take a step to the right. If you ask him if it's your right or his right, he'll tell you he doesn't care. Simply take a step to the left or right, and talk to the ghost. He is satisfied (or bored) and lifts his barrier. Again, you see a cutscene.
Spirit of the Mind
An animated book attacks!
Now, go to the final ghost, it's in the house south of Sir Prysin's home. You'll see a bookcase moving at the south wall. Investigate the bookcase. An animated book will fight you. Kill it. The final ghost will appear. He asks you some trick questions, all options will work (since there are no true 'answers' to his questions). After that, he lifts his barrier.
Dealing with Delrith
You can now go back to the crypt underneath the church and take Silverlight. Go to the south-east part of the city. If you try to go through the barrier, Evil Dave won't let you for not being in his "evil" cult. To get in, you'll need to have an "awesome black weapon and robes". There are some black mushrooms nearby Evil Dave, so grab one and use it on Silverlight. To get black clothing, go to Thessalia and buy some black robes from her, which will cost you 90 zemomarks.
Drawing a circle of blood for Delrith.
Talk to Delrith. You'll find out that he actually wants to be defeated and go back to Infernus. He'll ask you to simply stab him. You will automatically stab him, but a bolt of lightning will deflect the attack. Delrith tells you that that was Agrith-Naar, and asks you to draw a blood circle. Get a vial, which you can find in the General Store, and fill it by a blood fountain, which you can find north of the Varrock Herald, at the steps up to Varrock Castle. Go back to Delrith. Your character draws the blood circle, and stabs Delrith.