The Slug Menace

Starting off
To begin, talk with Sir Tiffy Cashien in Falador Park. He'll tell you about the city of Witchaven, which has had some rather strange goings-on recently. He'll tell you to check in with their local agent in Witchaven, Col. Jake O'Niall. Give your Commorb to Sir Tiffy Cashien to upgrade it to Commorb v2, which will start the quest; then, head to Witchaven.

Witchaven
O'Niall sits on a rocking chair on the pier at the north-east corner of town. Talk to him, and he'll tell you that some of the people have been acting strangely recently, especially Mayor Eustace Hobb. Recently, he started looking rather sickly and working feverishly to restore the ancient, ruined shrine just west of town (between Witchaven and Ardougne). Before leaving, ask O'Niall about who the important people in town are, which are Mayor Hobb, Brother Maledict, and Holgart. Speak to each of them in any order.

The Mayor
Although you can speak to any of the important people in any order, you can start off by heading to Mayor Hobb's house. It's the one at the north-west side of Witchaven. Talk to him, and ask him about Witchaven or the shrine. When you are done speaking to him, don't go anywhere as Savant will stop you. She'll ask you to scan the mayor, so click on the Commorb to scan. Once she's finished speaking, go to the church, and talk to Brother Maledict.

Brother Maledict and Holgart
Brother Maledict will mention that he's a bit worried about some of his congregation, who have begun to act a little bit strangely lately. After asking him any of the available options, walk a little bit north, and talk to Holgart, who'll tell you that the mayor recently took his boat and gave it to another man named Jeb, standing near by.

The Shrine
Report back to O'Niall again to tell him what you have found out so far. He'll suggest you go look into the shrine that the mayor is so interested in.

Walk a little ways west of town, and climb down the stairs to get into the shrine. Once you're inside, look at the minimap. You can see a red portion of wall indicating where there's a false wall that can be pushed open. Do so, and you'll be let in to a new area. Inside are some aggressive level 45 giant lobsters in a winding tunnel. Walk through it, or take the Agility shortcut to get to where there's an imposing door blocking your way. Try to open them, and Savant will contact the Commorb to tell you about the strange markings on the door. ( Warning: You will be dealt 5 damage for this, so make sure you have above 5 Hitpoints!) Scan the Commorb for her. She can't read them and suggests you take them to Jorral to try to find out what they mean. Pick up the dead sea slug off the floor before leaving and head to Jorral's house, which is located at an outpost just north-west of West Ardougne.

The Pages
Now, go talk to Jorral, who is at his Outpost. After some coaching from Savant on what to say, Jorral will translate the message for you. As you leave the building again, Savant will contact you again to let you know just what this new revelation entails. Head back to Witchaven again, and report in with O'Niall. He'll suggest you go talk to the priest and see if he knows anything about how to open the door. As you approach the church, you'll see a brief cutscene of the mayor leaving the church and when you go in, the priest will be looking rather ill and acting strangely. Anyway, when you ask him about the door, he'll tell you that he has a book that might be of use, but the three most important pages are missing.

For the first one, go to the mayor's house and look in his desk. The second one will be given to you by Ezekial Lovecraft, the owner of the Fishing store, Lovecraft's Tackle, when you talk to him. For the last one, go talk to O'Niall again. He too has taken the same look as the rest of the villagers. He'll give you the last page, but unfortunately, it has been ripped into three pieces. You, therefore, have to have five open spaces in your backpack for two whole pages and three pieces of the third page.

Try to put the pieces together with swamp paste, and Savant will stop you - the oily tar would ruin the paper. Instead, you should try to make a glue out of the slugs. Travel to the platform by talking to Jeb, and talk to Bailey on the platform, the man on the ground floor of the building. Give him the dead slug you got earlier from the shrine, and he'll give you the glue you need. (Note: The live ones will bite you.) Now, use the glue on the pages and a puzzle will pop up.

You can move the three pieces around using the buttons on the side, turn them, and even flip them over. The easiest way to tell if you've got them all on the right side is that one side is lighter than the other. It doesn't matter which way you have it, but the light side has an image on it. When you put the pieces in their correct place, the puzzle will close automatically. The picture will give you a general idea of how they should look.

The Imposing Door
Now, to open the final door. To do the next part, you'll need to get your chisel and at least five rune or pure essence. Right-click on each page, and select the options to make the runes into the shape of air, water, fire, earth, and mind. It is possible to fail this and break a rune, and be careful not to use the chisel on the essence, or you get fragments. Now, you need to go to each of the odd shaped essences' respective altars and use them on them (e.g. earth on the earth altar, fire on the fire altar, and so on). It is also possible to fail to enchant the shaped essence at the altar, thus destroying it. Keep your pages, chisel and spare essence handy as you make your way to the various altars. Using an Amulet of glory and the Abyss will shorten this process.

When that's done, head back to Ardougne and prepare for a fight; bring your newly created runes and Commorb. Then, go down into the shrine again. Use each of the runes you made on the door to trigger another cutscene. You see that Mayor Hobb tricked you into opening the door so that Mother Mallum could escape. After that, you'll have to fight a level 62 Slug Prince. It isn't that hard to kill, but the only trick is you can only use Melee. Arrows and Magic bounce right off of it. Its attacks drain your Prayer by the amount of damage dealt. Once you kill it, you'll be treated to another cinematic, after which you'll be teleported to Falador Park, just steps away from turning the quest in by talking to Sir Tiffy. Congratulations!

Reward
Promotion to Proselyte among the Temple Knights. This means you can buy and wear new Temple Knight armour with a greater Prayer bonus than Initiate. You can buy each piece individually, or buy the armour pack which, when opened, gives you the plate, helm, and legs/skirt all at once. The pack is worth 25,000 coins.
 * 1 Quest point
 * 3500 Crafting experience
 * 3500 Runecrafting experience
 * 3500 Thieving experience
 * Buying the upgraded armour in separate pieces cost more: the helm costs 8,000 coins, the platelegs and plateskirt cost 10,000 coins, and the platebody costs 12,000 coins for a total of 30,000 coins. Buying the pack, however, costs 25,000 coins.

Trivia

 * Mayor Hobb is currently the only known human victim of the sea slugs' control who achieved metamorphosis into a more aquatic creature, likely due to his long time possessed.
 * Hobb's beard's style changes after he is possessed.
 * Col. Jake O'Niall may be a reference to Stargate's Col. Jack O'Neill, both seem to have an affinity for fishing. Jake is also "semi" retired (while still retaining security clearance), as Jack was several times throughout the show. Jake also gets possessed by a sea slug, which seems to be an obvious reference to the Goa'uld, a parasitic race of worm-like aliens that take over a host completely.
 * Ezekial's last name is a reference to the famous American author of horror, fantasy and science fiction, Howard Phillips ("H.P.") Lovecraft. Ezekial mentions his family have always been either bait sellers or writers, and mentions his relative Howard. This is further confirmed by the nature of the quests he's involved in.
 * Instead of "Trade", like all other shop owners, Ezekial has a right-click option "Shop" that opens the shop's window.
 * Ezekial's favourite food is tuna, which is made clear in RuneScape 3's Salt in the Wound, for when fed any tuna-related item (such as a tuna potato with sweetcorn) he will exclaim "Tuna! My favourite!" and is healed a much higher amount of Hitpoints than normal.
 * Maledicts are evil sayings or doomsayings, which is ironic since Brother Maledict worships Saradomin.
 * Maledict derives from the word "benedict", meaning "good blessings", in which "bene" means "good", deriving from Italian. In Italian, "male" means "bad", so maledict literally means "bad blessings".
 * The scarf around Maledict's neck gets a gold trim when he is possessed.