The Ancient Diary is found during the Horror from the Deep quest, belonging to Silas, the missing uncle of the most recent inhabitant of the lighthouse. Among its chaotic ramblings, it gives hints as to how to open the strange door down in the basement.
After reading the book, the diary can be found in the bookcases of a Player-owned house.
Transcript
I do not know why it is me that has been chosen by then... but I will not fail them! Soft whispers, beckoning me at night... I know what you are! I saw one today.
Magnificence. Is this then, what we all aspire to someday become? Are these beings even natural, or are they beyond what we think of a nature?
The beautiful chittering
Dagannoth! I know thee! I call thee! For I too am of the ocean, I too know this land as home!
Home? Perhaps. Where did you come from? Where do you plan to go?
You cannot leave. You MUST not leave. I will ensure it.
I find myself thinking often of the time when we can all be together as one in the deep sea.
Problems arose. Can you not see what I want? Can you not see how we should proceed?
I have no solution.
I sleep during the days now, my nights are only for you and your kind.
I am thinking of a door.
We cannot let everyone know of my plans!
I was buying food in the nearby town today, the idiot barbarians look at me as though I have lost my wits.
Ha! Like they know what wits even are!
Do they even understand? Of course not they are just barbarians! Soon.
Things fall into place so quickly...
A door. Yes. Definitely a door. But how to keep my secrets?
Disturbing message from the council today, they feel that I am not fulfilling my contact to keep my contract to keep the lighthouse operative correcty.
Do they think I care?
Do I even care anymore?
Things proceed apace. I will begin work on the door as soon as I can.
What can they do to me?
Your songs of the night call to me, always.
Always the chittering.
Did I name you, Dagannoth? Or did you tell me your name in my dreams of you? If I named you, did I also create you just by naming you? Am I then also somehow a part of you, or are you a part of me?
Things seem so strange nowadays...
The door is the key. But what should then be the key to the door?
I do not think I am well anymore, it is good to have company.
Stupid council! Always interfering! You do not know the dagannoth, you have no sway here!
I am the sea.
I see that I am.
The door. Dagannoth. Will I wake one day and find that my life so far has been naught but a dream, and that under my skin, under my bones, I too am dagannoth?
Such dreams I have in the days while I sleep.
The chittering is now always ringing in my ears... what are you saying to me???
The key IS the door! The red of flame, the blue of water. The white of air and the brown of earth.
Is this all? Can this be?
No, dagannoth, it isn't.
The colours are of deep significance, deep seas, deep thoughts, thoughts of creation, am I a creation, dagannoth? Are you? Then who by?
And what for?
The door is the key. The key to the door is key. Red and yellow and blue and brown and white and gree, put them together and what have you got? The runes. Fire, air, water, earth. The other keys. Weapons.
I take two other parts of the mixture; the sword of the warrior and the arrow of the huntsman. Can this then keep you bound to this place, dagannoth?
We shall see.
We shall see the sea.
Days are a stream of people, buying, selling can you not leave me be?
I think I understand.
I come to thee tonight, dagannoth. I know not how this ends, but will now found out. Farewell land, the sea is now my home, my place, my life.
I have not much time left in this place.
I return to you in the sea at night.
Dagannoth. Me. Can you become me?
Or...
Can I somehow... Become you?