Horror from the Deep

Official quest description: The lighthouse protecting Kandarin's north western coastline has mysteriously stopped operating, and contact with the lighthouse keeper Jossik has been lost.
The Council would greatly appreciate it if somebody could discover for them what has happened to this most vital landscape feature.
Release date: 17 November 2004
Start: Speak with Larrissa, who is outside the Lighthouse, north of Barbarian Outpost.
Difficulty: *****
Length: Short
Members only: Yes
Requirements:
35 Agility
must have finished Barcrawl
able to defeat a difficult level 100 monster.
Items needed:
1 fire rune
1 water rune
1 earth rune
1 air rune
1 sword of any type that you are willing to lose
1 arrow of any type (except ogre) you are willing to lose
molten glass
tinderbox
a hammer
60 steel nails
2 planks (can be obtained from Barbarian Outpost)
1 swamp tar
(games necklace is optional and very helpful during the quest).
Must be able to defeat 2 level 100 daganoths.
Monsters to kill:
level 100 Dagannoth
the Dagannoth Mother
Spoiler warning: Plot and/or ending details follow.

edit Walkthrough
Although this quest is short, it is made troublesome by two hard monsters to be defeated, the Dagannoths.
[edit] Getting started
A Camelot teleport or Games Necklace will prove useful during the quest if several trips to the Lighthouse are made.
Start by speaking to Larrissa north of the lighthouse. Items to take with you now are: 60 steel nails, hammer, swamp tar, tinderbox, molten glass, 1 fire rune, 1 water rune, 1 earth rune, 1 air rune, 1 arrow, and 1 sword (last six items cannot be retrieved).

[edit] Fixing the Lighthouse

The lit lighting mechanism
After speaking to Larrissa, go south, jump across the basalt platforms and go to the Barbarian Outpost.
Talk to Gunnjorn and get the lighthouse key; you can get your two planks here, which respawn north of Barbarian's Outpost. To be able to enter the agility arena and talk to Gunnjorn you must have completed the miniquest Bar Crawl.
Use both planks on the broken bridge east of Larrissa.
Note: Even before fixing the bridge you can cross doing a small jump, so just hop to the other side and fix the other part of the bridge.
Go and talk to Larrissa again.
Go up to the second story. Search the bookcase and take all the books available, flip through the lighthouse manual and the Ancient diary. (The journal plays no part in the quest).
Go up one more level, use the following items on lighting mechanism in this order: swamp tar, tinderbox, molten glass. The lighthouse will now be fixed.
[edit] The Door

The fully filled doorGo to the bottom floor of the lighthouse then use all of your basic elemental runes on the door, (fire, water, earth, and air)
Use your sword (any type, but you will not get it back), and your arrow (any type, but you will not get it back).
If you brought your stuff for the fight, then you can save some time. If you hadn't, teleport to Camelot or walk to bank.
Get good armour (Rune or better), or mage armour (Mystic or better) bring many mind, chaos, death, or blood runes and enough runes to cast several of each elemental spell. (wind, water, earth, and fire), get a good weapon for melee, and some ranger equipment. Equip a ring of life or bring a one-click teleport just in case things get twisted.
[edit] The Fight
Go though the door and down the ladder.
Talk to Jossik, be prepared to fight two Dagannoths.
The first one is easy, put on protect from melee and range or melee it, because you want to save your runes for the mother.
Then right after you defeat it, there will be a cut scene, and you will fight a harder monster.(be careful she can max up to 24's on lower levels with her range attacks)
You should attack the Dagannoth Mother by colour if you want to do any damage more than 1 or 2.

The Dagannoth Mother
White means you should use wind spells.
Blue means you should use water spells.
Brown means you should use earth spells.
Red means you should use fire spells.
Orange means you should use melee.
Green means you should use ranged.
!NOTE! You can kill the baby dagannoth, then run away after you have talked to Jossik again, and still battle the mother next time you go there(even if you log out).If you take too long to kill the dagannoth mother it will disapear and you will have to restart the fight.
[edit] Note
If you cannot see the colour changes very well, look at the chat box. Using the wrong attack does no damage to it. Also there is a safe spot, if you do it right you can get the mother stuck behind a rock were you can stand behind and not lose any hit points.
The best way to defeat her is to ignore the ranged and melee forms, since she has too high defence against those attack types for them to be worthwhile. Use an air staff and switch spell types in the correct order.
She can detect prayer (so bring prayer pots), and will automatically use the opposite attack type when you use prayer. By default, she ranges at a distance and melees up close. Her melee attacks are stronger, so Protect from Melee is recommended. Also if you "don't" have ranging items, you can keep clicking under her so she doesn't hit you, or simply hide behind a stalagmite.
Do not bring a summoning familiar, as it will rarely ever hit because when fighting, the familiar won’t alternate attacks and it will simply waste time and resources to switch between familiars every thirty seconds. (Item carrying familiars are very useful for food or potions!)

[edit] After the fight
After you defeat the Dagannoth Mother, the quest is finished.
As a reward from the Dagannoth you get a rusty casket. Go to the second floor of the lighthouse and show it to Jossik. If you had a full inventory when defeating the Dagannoth Mother, you will not receive a rusty casket. Instead, you must talk to Jossik, who will have picked it up for you.
You can now choose one of the three damaged prayer book you want (Zamorak, Guthix, Saradomin) from Jossik.