Warlock subclass
Fathomless
Tentacle attacks, ocean adaptation, and deep-sea control.
Fathomless starts by changing the normal Warlock play pattern through Expanded Spell List; Tentacle of the Deeps; Gift of the Sea, giving the subclass its first clear mechanical identity.
Later features such as Oceanic Soul; Guardian Coil, Grasping Tentacles, Fathomless Plunge expand that identity at higher levels, so the subclass has a clearer role in combat, exploration, utility, or support.
Expanded Spell List; Tentacle of the Deeps; Gift of the Sea
1st level
Gain ocean-themed spell 
D&D Portal - Spells Spellcasting rule Spells is a spellcasting rule used by spells,
class features, and magical effects. Full link: https://dnd-portal.com/rules/spellcasting/spells/ options, summon a bonus-action tentacle attack that slows targets, and gain swimming 
D&D Portal - Swimming Movement rule Swimming is a movement rule used during
exploration, encounters, and combat. Full link: https://dnd-portal.com/rules/movement/swimming/ and underwater breathing.
Oceanic Soul; Guardian Coil
6th level
Gain cold 
D&D Portal - Cold Damage type Cold is a damage type used by attacks,
spells, class features, resistance, immunity, and vulnerability. Full link: https://dnd-portal.com/rules/damage-types/cold/ resistance and underwater communication; use the tentacle as a reaction 
D&D Portal - Reaction Action rule Reaction is an action economy rule used to
describe what a creature can do on its turn. Full link: https://dnd-portal.com/rules/actions/reaction/ to reduce damage.
Grasping Tentacles
10th level
Cast Evard’s Black Tentacles without a slot once per rest and gain stronger concentration 
D&D Portal - Concentration Spellcasting rule Concentration is a spellcasting rule used by spells,
class features, and magical effects. Full link: https://dnd-portal.com/rules/spellcasting/concentration/ while casting it.
Fathomless Plunge
14th level
Teleport yourself and allies to a body of water you have seen within a vast distance.
FAQ
What is the Fathomless subclass?
Fathomless is a specialised subclass option that builds on the features and progression of its parent class.
Read more: Fathomless 
D&D Portal - Fathomless Warlock subclass Tentacle attacks, ocean adaptation, and deep-sea control. Full link: https://dnd-portal.com/classes/warlock/fathomless/
How does the Fathomless subclass work?
The Fathomless subclass changes the character’s play style through its own feature progression.
Read more: Fathomless D&D Portal - Fathomless Warlock subclass Tentacle attacks, ocean adaptation, and deep-sea control. Full link: https://dnd-portal.com/classes/warlock/fathomless/
What features does a Fathomless get?
The subclass page lists the features gained by a Fathomless and explains their effects.
Read more: Fathomless D&D Portal - Fathomless Warlock subclass Tentacle attacks, ocean adaptation, and deep-sea control. Full link: https://dnd-portal.com/classes/warlock/fathomless/
Is Fathomless a good subclass?
Whether Fathomless is a good choice depends on the role, abilities, and play style you want for the character.
Read more: Fathomless D&D Portal - Fathomless Warlock subclass Tentacle attacks, ocean adaptation, and deep-sea control. Full link: https://dnd-portal.com/classes/warlock/fathomless/
How do you play a Fathomless?
Use the subclass features to specialise the normal class play pattern and support the role you chose.
Read more: Fathomless D&D Portal - Fathomless Warlock subclass Tentacle attacks, ocean adaptation, and deep-sea control. Full link: https://dnd-portal.com/classes/warlock/fathomless/