From LSWiki

Description

The Deep Ones are initiates of the worship of Great Cthulhu, as handed
down from Father Dagon and Mother Hydra.  The spark of insight that makes
a Deep One can only be sustained by one of the true people, the Urlnu.

How to Join

Father Dagon and Mother Hydra are two immense Urlnu living beneath R'lyeh. In their presence, state 'I wish to become of the Deep'.

Note: Only the laireang'th people may receive initiation. Skaven isn't one of them. Pack of racists.

You murmur, "I wish to become of the Deep"
>
    You are now a first-degree specialist in Ngaathgl. [event logged]

    Dagon locks eyes with you for a long moment.

    You feel a sense of terrible knowledge flowing into you, and the supremacy
of Great Cthulhu over all the things of the world becomes mind-shatteringly
clear to you.

    Lusthalyo has become a Deep One.

    Type 'help Deep Ones' for more information.

    Hydra murmurs, ~ You are already a Deep One. ~ to you.

Specialty Access

As a Deep One, you gain no new special powers, just a channel, trainers and some specialization access:

Skill             Max   Min 
---------------------------------
Centering         +2     
Channeling        +2     
Cold tolerance    +2     
Elder lore        +2     
Ngaathgl          +3    +1
Subordination     +2     
Theology          +2     
Torture           +2     

Despite being a Deep One, R'lyeh NPCs will still attack you.