Guide by Failboaticus

Requirements:
To do this quest, you need to have completed the Barcrawl and the Scorpion Catcher quest

Skills Needed:
35 Agility, whatever you need to kill a level 100 monster.

What You Need:
60 steel nails, 2 planks, hammer, tinderbox, any arrow, any sword, 1 of each elemental rune (air, water, fire, earth), molten glass, swamp tar

Recommended Items:
Prayer Potions, food, armor/ammo for all combat styles, additional elemental runes (air, water, fire, earth)

Where To Start:
You can start this ques my talking to Larrissa by the Lighthouse north of Barbarian Assault


Starting Off the Quest
To start, head over to the lighthouse to the north-west of the Barbarian Outpost. You can get there by jumping across the Basalt Rocks on the coast, until you reach an island. Go north from there and talk to Larrissa. She wants your help, surprisingly enough. She complains about her missing boyfriend, and asks if you could talk to her cousin.

Lighthouse Repairman
Now, go back to the Outpost, and go in the gate. Climb through the pipe to the south and talk to a man called Gunnjorn the Barbarian to the south-west. He gives you a Lighthouse Key, which lets you in to the Lighthouse, obviously.

For this part, you need your 60 nails, 2 planks and hammer to fix the bridge. Go to the Lighthouse and go south-east along the path to the broken bridge. Use one plank with each side to repair, and head back to Larrissa.
After a conversation with her, go inside the Lighthouse and up the staircase. Search the bookcase for 3 books, and read them all. Go up those stairs and use your Swamp Tar with the Lighting Mechanism. Use a tinderbox on that to light it, and use the molten glass to get yourself a fixed light!

The Dagannoth
Climb down the ladders back to the main floor, then climb down the iron ladder to the basement. On the north wall, there should be a Strange Wall. Use one of each elemental runes and your arrow and sword on it to open it. You do not get these back. Now, open the Strange Wall and climb down that ladder to a cavern. Talk to Jossik, and he tells you that a monster attacked him! Now you will be attacked by a level 100 Dagannoth. Once you kill him/her, you will meet the mother. She is a mean, level 100 Dagannoth Mother.

That Mother...
The Mother fights with all 3 combat styles! This makes it difficult to attack her with a single combat style. While fighting her, she will change color. when that happens, a message appears in the chatbox, telling you what she is weak against! these are:
Orange: Melee Attacks
Green: Ranged Attacks
White: Air Spells
Blue: Water Spells
Red: Fire Spells
Brown: (you guessed it...) Earth Spells


The Mother attacks most often with melee, so Protect from Melee prayer is very useful, because her melee attack has proven itself to be her most powerful. But be on your toes, and ready to switch your prayer to match whatever style she uses.
To safespot, wait behind a stalagmite (stalactite?) and wait for her to be vulnerable to a mage / ranged attack, with your prayers on.

Finishing the Mother
When you kill the Mother Dagannoth, she will drop a rusty casket, and you find yourself outside of the strange wall. Quest Completed!
You can bring your casket to Jossik, who is upstairs in the library. He asks you what it says, and then you choose a god: Saradomin, Guthix, or Zamorak. Once you choose a 2nd time, you get a damaged book of that god.

Rewards:
After visiting Jossik, he gives you a damaged God Book, which, when wielded, gives you a +5 prayer bonus.
4,622 Magic, Ranged, and Strength experience
2 quest points