To begin the quest, speak to Ceril Carnillean who I located in his house south of King Lathas' Castle. Ask him what's wrong and he will explain that cultists keep breaking into his house. Ceril goes on to explain that they've stolen a piece of armor and have broken into his house 4 times afterward, but have not taken anything...
Head south of Ceril's house, crossing the bridge along the way. The Clock Tower should be nearby. From the Clock Tower, head east until you see a cave entrance. Enter it. You should see a person named Clivet next to a raft. Talk to him. Your character will ask if Clivet knows anything about the Carnilleans or the Cult and will be told to go away. Your character will keep pressing Clivet about the Cult and eventually Clivet will talk about how the Ceril is not the rightful owner of the house; the true owner is the Zamorakian Mahjarrat, Hazeel! Clivet also go on about how Hazeel was forced out of his house by the Carnilleans and how the Cult is preparing for Hazeel's return.
You will eventually be able to choose joining the cult, or continuing to help the Carnilleans. Your choice will affect how the rest of the quest plays out, but does not affect the reward.
If you're helping out the Cult, click here or scroll down to the Helping the Cult section.
If you're continuing to help the Carnillean's, just read this next section.
Helping the Carnillean's
Following the map turn the valves in their proper direction and then return back to the cave.
Once you are at the Cult's hideout, talk to Alomone (the only one without a staff). You will then learn that the Carnillean's butler is part of Cult as well! He will then attack you. Kill him and pick up the Carnillean Armor.
Leave the hideout and report back to Ceril. Ceril will be glad to have his armor recovered, but does not believe you when you tell him that Butler Jones is in league with the Cultists. Ceril will hand you 5 gp and 1995 gp to Jones who smirks at you.
Erm, quest Complete?
Finding the dirt on Jones
To find evidence against Jones, look in the wardrobe in the upstairs room that you talked to Ceril. You will find a bottle of rat poison and an mysterious amulet which you show to Ceril. Jones will claim that it's for killing rats and then you will show Ceril the amulet. Ceril will then call the guard to arrest Jones who will take him to Port Sarim Jail...
1 Quest Point, 1,500 Thieving XP, 2,000 Coins
Helping the Cult
Clivet will give you poison asking you to poison one of the family members. Back in the house, go down the ladder to the basement. Use the poison on the range, then go back up and talk to Ceril to find out that their dog is dead.
Return to Clivet, and he will give you the mark of Hazeel, and ask you to find the hideout. Find and turn the valves as noted in the below map.
Go back in the cave and board the raft. Talk to Alomone, and he'll ask you to find a scroll in the house. Return to the house and talk to Butler Jones. Go down to the basement again and search the crates for a key.
Go to the top floor and knock on the odd wall along the northern wall. Open the chest to get the scroll then return to Alomone. Quest complete!