Horror From The Deep
Difficulty: Easy
Length: Short
Items Needed to Complete Quest: 2,050 coins
Recommended levels for Quest: 43 Prayer, 50 Attack, 50 Strength
Items Recommended for Quest: Prayer potions (4), Attack potion (4), Strength potion (4), Food and Armor.
Reward: Zamorak Book, Guthix Book, Saradomin Book
Start Point: The Lighthouse, found in Slayer Teleports
Start: Speak to Larrissa.
Instructions:
Finding Larissa
Use your Slayer Teleports to teleport to the Light house, then speak to Larrissa.
Beginning the Quest
Larrissa will inform you that her boyfriend Jossik has went into the Light house and hasn't returned, choose to help her find Jossik.
Finding Gunnjorn
You need a key to enter the Light house, Larrissa advises that you find Gunnjorn in the Varrock Wilderness Pub, as he has a spare key. Use your City Teleport (not avaiable on the Lunar spellbok) to teleport to Varrock. Walk North-east to the Wilderness Pub and speak to Gunnjorn.
Upon talking to Gunnjorn, you'll find out that he doesn't feel like helping you. You'll have to do something to lighten him up a bit.
Lightening Gunnjorn up
Talk to the Bartender to buy a beer for 50 coins.
Give the beer to Gunnjorn, this will make him drunk. He will now give you the key if you give him more coins to pay for more beer.
Choose to pay him 2000 coins, and in return he will give you A key.
Note: If you do not choose to pay the 2000 coins, you will have buy him another beer or he will not talk to you.
Using the Key
You should now had back to the Light house and speak to Larrissa. She will inform you that you should talk to her when you're ready to go inside the Light house.
You should take this time to prepare yourself to battle the boss. Even at low levels, this boss isn't much of a challenge. Your inventory should look something like this:
This is the recommended low-level gear, any gear better than this will also work fine:
Fighting the Mother
After gearing up, head back to the Light house. It is recommended that you take a sip of your potions and Protect from Melee before speaking to her. Once you're ready, speak to Larrissa. Once you've been teleported in, you must kill the level 100 Dagannoth Mother.
All you need to is Protect from Melee the entire fight, and you shouldn't get hit.
Congratulations! 
Rewards
Now that you've completed the quest, you should go upstairs in the Light house and speak to Jossik for your rewards.
Upon going up, you will find Jossik. Speak to him and you will need choose between 3 different books (you can take all 3).
Note: If you choose a book, you can equip it and ask for the same book, and you will be to receive two of them. This works for all 3 books if you want doubles. 
Additional Information and Book Stats
Unholy (Zamorak) Book
Holy (Saradomin) Book
Book of Balance (Guthix)
Credit to Pixel