This quest has a quick guide, found here, which briefly summarises the steps needed to complete the quest.

Walkthrough

Start point:	Speak with Larrissa, who is outside the Lighthouse, north of Barbarian Outpost.
Member requirement:
Official difficulty:	Experienced
Length:	Short (Long if you haven't already done the Bar Crawl miniquest)
Requirements:	Quests: Barcrawl 35 (boostable) Agility 43 is HIGHLY recommended. Ability to defeat two Level 100 monsters, the second one being quite tricky. Level 13 magic is not required but needed to use elemental spells. Decent ranged and melee levels.
Items required:	Items from the toolbelt do not need to be listed unless that version won't work, or aren't automatically added to it. 1 Fire rune 1 Water rune 1 Earth rune 1 Air rune A sword of any type that you are willing to lose An arrow of any type (except ogre and training) you are willing to lose, Ice Arrows will work Molten glass Tinderbox (can't be from the toolbelt) A hammer (can't be the Golden hammer) 60 steel nails 2 normal planks (can be obtained from Barbarian Outpost, north of the agility course) 1 swamp tar A melee weapon, ranged equipment, and runes for all four elemental spells to switch amongst in combat Recommended: Games Necklace and quick teleport tabs are very helpful to reach the Lighthouse during the quest. Houses by Rellekka are also helpful transportation methods.
Enemies to defeat:	Level 100 Baby Dagannoth Level 100 Dagannoth Mother

Getting started

Travel to the Lighthouse by using an enchanted lyre, a Games necklace teleport to Barbarian Outpost, a Teleport to House (if your house is in Relekka) or any other method that you wish. Start the quest by speaking to Larrissa, who is standing just oustide the lighthouse gate.

Fixing the Lighthouse

• 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 area and talk to Gunnjorn you must have completed the miniquest Bar Crawl.

• Use both planks on BOTH sides of the broken bridge located east of Larrissa, shown in the map above(You will need 60 steel nails for this part.)

• You can cross the broken bridge by doing a small jump, so just hop to the other side and fix that part of the bridge.

• Go and talk to Larrissa again.
• Go to the first floor, search the bookcase and take all the books available, flip through the lighthouse manual and the Ancient diary. (Jossik's Diary plays no part in the quest).
• Go to the second floor, use molten glass, swamp tar (not swamp paste), and a tinderbox (cannot use toolbelt) (Dwarven army axe can work here) on the lighting mechanism. The lighthouse will now be fixed.
• Go talk to Larrissa once more.

The Wall

Using the ladder, go down to the basement of the lighthouse. You will find a small cavern entrance with a Strange Wall, which has six empty slots fitted into its frame.

• Use all of your basic elemental runes on the wall, (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).

Warning: This is your last chance to prepare for the fight.

Preparation

Inventory

If you haven't prepared for this fight with armour and runes for casting magic, teleport to any place with a bank to ready yourself. If you lack a teleport (other than home teleport), Camelot is the closest walk. If you only brought a games necklace, then use it to teleport to the Gamers Grotto or Burthorpe Troll Invasion and bank there. While you are there, pick up any quick teletab to any city or your POH, just in case you are close to death, to save yourself.

• Get good armour (Rune or better), Dragon hide armour, or mage armour (Mystic or better), bring many combat runes (chaos, death, or blood runes) to cast several of each elemental spell. (wind, water, earth, and fire), get a good weapon for melee, and some ranged equipment. If possible, equip a ring of life and bring a one-click teleport just in case things get messy.
• Stock yourself with Prayer potions.
• Sacred clay armour (and perhaps a Sacred clay weapon) can be useful for the fight. It is the equivalent of level 40 ranged, melee, and magic gear. What makes it especially useful is that, when right clicking on one piece to transform it, the whole set is transformed (when worn).
• Void armour can also be used due to the ease of switching combat styles (by changing helms), but it is not recommended due to its low defence bonuses.
• Splitbark armour may be useful because it has decent melee defences and very few negative bonuses.
• One way of setting up your armour and inventory would be your best ranged armour, A Mystic Air Battlestaff, the Dragon Dagger P++, and a good melee weapon such as the Abyssal Whip.
• For lower levelled players, the following inventory is recommended: Super Strength potion (4), Super Attack potion (4), a few Ranged potions, lobsters (or monkfish), a prayer potion, a good melee weapon with a special attack, and a teleport tab if you don't think you will survive.
• As dust runes (earth + air) are currently very cheap, a good idea for the mage part is to use a steam mystic staff (water + fire) and a large number of dust runes. This allows any elemental spell to be cast without changing staves, only your autocast.

Combat

You can kill the Baby Dagannoth, leave after you have talked to Jossik again, and still battle the mother next time you go there (even if you log out).

If you cannot see the colour changes very well, (while fighting) walk past the monster to see its back, or look at the chat box. There would be a message "The Dagannoth Mother turns (colour)..." Also, you can change the graphics to "safe mode" to brighten the scene, allowing you to see her colour clearly. Using the wrong attack does no damage to it.

By default, her first form is always white. The colour changes of the Dagannoth Mother seems to be totally random except for her first colour. (e.g.: she starts off white, turns red and turns white again).

You can safespot her on the stalagmites at the far end of the room.

If you stop attacking her at any point or take too long to finish her off, there is a possibility that she will disappear and you will have to start the fight again.

She can detect prayer and will automatically switch to another attack type when you use prayer. By default, she ranges at a distance and melees up close. Her ranged attacks are stronger and hit twice, so Protect from Missiles/Deflect Missiles is recommended. Also, if you don't have ranging items, you can keep clicking under her to prevent her from hitting you.

If a player has reasonable defence, they could consider not using protection prayers at all but use attack bonus boosting prayers instead, because the Dagannoth Mother has a rather poor accuracy.

Sometimes, due to some bugs, the system treats you as still fighting the first dagannoth. When this happens, simply climb up the ladder, back down, and talk to Jossik again.

Strategies

The Dagannoth mother is not as hard to defeat as she may seem. All you need is a decent combat level (around 50) and prayer (at least 43). Bring some prayer potions and LOTS of food. Make sure you can cast all the elemental spells (air, water, earth, and fire) with some level of proficiency.

Some of the ways to defeat the Mother are:

• 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. If you use this method it is recommended to stand out of melee range or in a safespot and use the Protect from Missiles prayer. If you have a Magic level over 40, you can use bolt spells with their chaos gauntlets which hits for reasonable damage.
• Another way to defeat her is by using Protect from Missiles, the safespot, runes or staves for each of your four highest elemental spells, along with a crossbow like the Dorgeshuun c'bow. 3-4 doses of a Prayer potion may be enough. Whenever the Dagannoth mother changes colour, click at your feet (to stop wasting runes or ammo), then switch weapons or spells and continue attacking. When she changes to orange (melee), equip an air staff and cast Wind Rush until she changes again, because if you don't attack, she will get bored. Using this method, no defensive armour is necessary, provided that your prayer does not run out and you don't accidentally move from the safe spot.
• Another method that could be used involves standing behind Jossik while fighting the Dagannoth Mother with mage and range. In this position she cannot attack you. However, if you take too long in killing her, she will vanish because she cannot engage you in combat and you will have to start over in killing her.
• It IS possible to attack the Dagannoth Mother using only melee. The best course to do this is to use Protect from Range prayer and use the Dragon Dagger (P++). Most of the direct hits will not affect the Dagannoth Mother, but the poison damage will work. Using this method though, the combat will take more time than using other melee weapons.

Get ready to face the Mother!

The Horror

• Go through 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.
• Right after you defeat it, a cutscene will show the Dagannoth Mother emerging into the cavern, and you will now need to fight a harder monster. (Be careful; she can hit up to 160 with melee and up to double 120s with ranged)
• You should attack the Dagannoth Mother by colour if you want to do any damage more than 10 or 20.

Finishing Up

After you defeat the Dagannoth Mother, Congratulations. You have survived the Horror from the Deep!...once.

As a reward from the Dagannoth you get a rusty casket. Go to the first 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.

Reward

• 2 quest points
• 4,662 Experience
• 4,662 Experience
• 4,662 Experience
• 2 extra spins on the Squeal of Fortune
• Access to the lighthouse general store and the Dagannoth caves underneath (There are level 74 and 92 Dagannoths. The level 74's use a ranged attack that has terrible aim, and the level 92's use their much more effective melee attack)
• The Dagannoth ribs can be obtained by killing Dagannoths after the completion of this quest to help complete the 2nd part of the Rag and Bone Man quest.
• A damaged prayer book of your choice: a Holy book (Saradomin), an Unholy book (Zamorak), a Book of Balance (Guthix). A Holy book gives +8 to all defensive equipment stats, an Unholy book gives +8 to all offensive equipment stats, and a Book of balance gives +4 to both.

WAIT! Before you leave the lighthouse, go to the first floor and talk to Jossik. He will help you see what is written on the box. It is very important that you make a choice and stick by it.

After collecting all the pages (1-4) of the book YOU chose, you can start the other three (a Book of War (Bandos), a Book of Law (Armadyl), or an Ancient book (Zaros).

Music unlocked

• Out of the Deep
• Lighthouse

Required for completing

Completion of Horror from the Deep is required for the following:

• Blood Runs Deep
• Recipe for Disaster/Defeating the Culinaromancer
• The possibility of making Illuminated god books

Trivia

• The quest was originally named Lighthouse Quest.
• Upon the completion of this quest, one's Adventurer's Log will read: "A foul and horrible creature dwelt below the lighthouse. I have slain it, but the lighthouse keeper, Silas, remains missing."
• According to Jarvald in Rellekka, the Water-birth island dungeon is filled with "dagger-mouths", which is how dagannoths got their name.
• When you first speak to Larrissa, she will say "Oh, thank Armadyl!" which may mean she is a follower of Armadyl, or it could just be an expression.
• The lighthouse user's manual mentions a service centre advertised in the Varrock Herald. However, there is no such advertisement in the Herald.
• During September 2009, there used to be a glitch in which you would kill the Dagannoth Mother but the quest would not be completed. This has now been fixed.
• As of today, there is still a known glitch that if you speak to Jossik and run during mid-sentence of his speech. The Mother will come out of the water ... but a second one may come out as well. This is a rare glitch.
• There is a glitch where, when you get the book and complete the quest, the quest complete doesn't come up but you still get the rewards and the quest points.
• There is a glitch where, when failing the first basalt jump on the lighthouse side, you can click the basalt rock and turn endlessly on the spot.
• When you see the "Quest completed!" screen, it does not show, "You have completed Horror from the Deep!", but it shows "You have survived the Horror From The Deep!" which refers to the tough monsters in the quest.
• It is impossible to obtain a book of Armadyl, Bandos, or Zaros as a reward for the quest. You can only choose to buy those books after the quest.
• There is sometimes a glitch where the option of Bandos, Armadyl or Zaros books do not come up. This has currently not been fixed.
• During the fight, if the Dagannoth Mother cannot hit you, she will vanish. You will then have to fight her again with her health replenished. This has made the quest somewhat harder. Currently if you get hit several times and then hide behind a block (for example wait till she gets green so you can range her) the Dagannoth Mother WILL NOT disappear and the quest can be done if you only use one type of attack.
• The name of one of the unlocked music tracks, out of the deep, may be a reference to "Out of the Blue" by Debbie Gibson, a popular late 1980's teen pop star.
• The name of the quest may be a a reference to the X-Com game "Terror from the Deep".