Horror From the Deep

Start: Begin by talking to Larrissa near the Lighthouse north of the Barbarian Outpost.
Skills: 35 Agility and the ability to kill a level 100 Monster.
Quests: The Barcrawl from the Scorpion Catcher Quest must be completed.
What You Need: 60 Steel Nails, 2 Planks, a Hammer, Tinderbox, Swamp Tar, Molten Glass, 1 Air, Water, Fire and Earth Rune, an Arrow and a Sword.


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Getting Started

First off, head to the lighthouse located northwest of the Barbarian Outpost by jumping across the Basalt Rocks on the coast until you reach a cliff-like island. Head north to the Lighthouse and talk to Larissa who asks for your help. Ask her what you can help her with, and she explains about her boyfriend being missing, and she'll tell you to visit her cousin to see if he can help.

Fixing the Lighthouse

Now head back to the Barbarian Outpost and go in through the gate. Now climb through the Pipe to the south and talk to Gunnjorn the Barbarian near the house to the southwest. He should give you a Lighthouse Key, which will allow you to get into the Lighthouse.

Now you'll need 2 Planks, 60 Steel Nails and a Hammer to fix the bridge. Planks can be found just to the north of the outpost, and Steel Nails can be either Smithed or traded. Go back to the Lighthouse and head southeast along the path until you reach a broken bridge. Use a plank with each side of the bridge to repair it, and then head back to Larissa.


After speaking to her, go inside of the Lighthouse and head up the staircase. Search the bookcase, take all 3 books and read through them. Go up the stairs and use some Swamp Tar with the Lighting Mechanism. Use a Tinderbox with the mechanism to light it, and then use some Molten Glass with it to fix the light.

The Dagannoth

Climb down the ladder to the main floor, and then climb down the Iron Ladder to find a basement. On the north wall should be a Strange Wall, which you need to use 1 of each Elemental Rune, as well as any type of Arrow and Sword with it to open. Remember, you won't get any of these items back later.

Now open the Strange Wall and climb down the Iron Ladder to find a cavern, as well as Jossik who seems to be injured. Talk to him, and he says that a monster attacked him. You'll then be attacked by a level 100 Dagannoth, and after killing it you'll be attacked by another, meaner, level 100 Dagannoth Mother.

The trick to defeating the Dagannoth Mother is that you can only hurt it with certain attacks when it's a specific color:

Orange - Melee attacks
Green - Ranged attacks
White - Air spells
Blue - Water spells
Red - Fire spells
Brown - Earth spells
Note: if you're colorblind or find it difficult to distinguish between the colors, the dagannoth's color change will be announced in the chat box.

Usually, the Dagannoth Mother will hit you with melee attacks, so it's a good idea to use your Protect From Melee prayer if you have it and bring several prayer potions. It may also Range at you for a short amount of time. Bring food, combat gear, Runes for attack spells, and Ranging equipment. You don't need to bring Runes or Ranging Gear, but it's helpful if you do.

After killing it you'll find a Rusty Casket, and you will find yourself just outside of the Strange Wall. Your quest is complete! You can bring the Rusty Casket to Jossik, who can now be found upstairs in the library. He'll ask you what it says, and you'll have the chance to choose 1 of the 3 Gods: Saradomin, Zamorak and Guthix. After choosing a 2nd time, you'll receive a Damaged Book of that God.


Rewards

Once you visit Jossik with the Rusty Casket and pick a God, you'll get a Damaged Book of that god, which when wielded gives +5 to Prayer.
4,662 Magic, Ranged and Strength experience
2 Quest Points