The Slug Menace/Quick guide

Starting out
Item required: Commorb
 * Talk to Sir Tiffy Cashien to get a Commorb v2.
 * Talk to Col. O’Niall in NE Witchaven. Chat option 3.
 * Talk to Brother Maledict, Mayor Hobb, and Holgart.
 * Talk to Col. O’Niall.

The shrine
Items required: None
 * Climb down the stairs just west of town.
 * Push the false wall (red on minimap).
 * Follow the path to the imposing door, using the shortcut if able, try to open it (must have a free inventory space).
 * Scan with the Commorb.
 * Pick up a dead sea slug.
 * Talk to Jorral in the outpost NW of Ardougne.

The pages
Items required: 1 swamp paste, a dead seaslug, and 5 free inventory spaces
 * Talk to Col. O’Niall.
 * Talk to Brother Maledict.
 * Go to the mayor’s house and search the desk.
 * Talk to Ezekial Lovecraft in the fishing store.
 * Talk to Col. Jake O’Niall.
 * Use swamp paste on one of the fragments.
 * Talk to Jeb to go to the Fishing Platform.
 * Talk to Bailey to the west, give him the dead slug.
 * Use the sea slug glue on a fragment, then solve the puzzle.

The imposing door
Items required: a chisel, 5+ rune or pure essence, the Commorb Quest complete!
 * Right click on the pages to shape essence into air, water, fire, earth, and mind.
 * NOTE: It is possible and likely to fail and break the essence.
 * Use the chiseled stones on their respective altars.
 * NOTE: It is possible and likely to fail this as well.
 * Use the handmade runes on the imposing door.
 * Kill the level 62 Slug Prince, only melee can damage it.
 * Talk to Sir Tiffy Cashien.

Reward

 * 1 Quest point
 * 3500 Crafting experience
 * 3500 Runecrafting experience
 * 3500 Thieving experience
 * 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.
 * 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 roots "male," meaning "bad," and "dicere," meaning "to say." "Maledict" therefore roughly means "evil sayer."
 * The scarf around Maledict's neck gets a gold trim when he is possessed.