Hadal Mind

Somewhere out there, a captain guides a ship through the dense magical rivers of the elemental chaos, teetering on the edge, but one moment away from the edge of reality. The captain does not sleep, for his attention alone separates the crew he guides from the abyss. One day, the captain would waver for naught but half a second, and his ship would fall off the edge. Moments later, the ship would re-emerge off the coast of Cape Sallin. The captain was fine, but he could hear the spirits of the crew that fell off the edge with him, they haunted him, till his very death.

Some say that upon his arrival in the afterlife, he was bound to the very ship he careened off the edge of existence. Forever bound to his duty, sailing the lost souls to their new life. But now, he had the powers of what he found off the edge of the world. This is of course speculation, from what I can find, this is nothing but a cautionary tale to never sail too far from home. There is no evidence of actual eldritch horrors or a supposed 'edge of the world', but that doesn't make the tale any less haunting.

-Grimoire Regalia               

Eldritch Plunderer

You can move and preform actions as normal while underwater. While underwater, you gain fish-like attributes including scaly skin and fins, that allow the ability to swim and breathe while underwater.

Abyssal Weaponry

You gain the ability to summon abyssal tentacles to strike at opposing creatures. On your turn, when you deal damage to a creature with your sneak attack feature, you can expend a use of this feature to cause shadowy tentacles to strike at two other different creatures within range. Roll a d20 and add your dexterity modifier and your proficiency bonus, if the result is higher then the creature's armor class, that creature takes necrotic damage equal to the sneak attack's damage. If you use this ability and there isn't two different targets within range, you can strike another creature twice. You can use this feature a limited number of times per long rest equal to your dexterity modifier + your proficiency bonus.

The range that your tentacles can lash out increases by 10ft when you reach certain levels. Increasing at 5th level (30ft), 11th level (40ft), and 17th level (50ft).

Sentient Horror

You can call upon the abyss to horrify another creature, sending them into a frenzied panic. As a bonus action, you can horrify another creature that you can see within 50ft. That creature must make a wisdom saving throw equal to 12 + your wisdom or intelligence modifier (whichever is higher) or become frightened of you for the next 10 minutes. You have advantage on persuasion and intimidation checks made against that creature. A creature repeats this saving throw at the end of each of their turns.

You can use this feature a limited number of times per short rest equal to your dexterity modifier.

Eldritch Lullaby

An soft lullaby echoes through your mind, you are unable to stop hearing it and sometimes hum along with it whenever you lose focus. You are immune to being charmed or frightened. You have advantage on wisdom saving throws.

Extraplanar Mind

The abyss has blessed you and granted you magical powers that transcend your mind to another plane. When you take this feature, you can choose an eldritch power from the choices below. Whenever you finish a long rest, or spend an hour to release your mind, you can change which eldritch power you have.

Thoughts of Madness

You open your mind to the elemental chaos and it's infinite madness. Whenever you cause a creature to be frightened, they immediately use their reaction to attack a random creature within their melee range.

Thoughts of Nothing

You blank your thoughts and allow your mind to float among nothing, your empty mind cannot be influenced. You gain 50ft of truesight.

Thoughts of Safety

You open your mind to the imaginary realms, dreaming of safety and prosperity. You and all your allies within 10ft are immune to psychic damage.

Thoughts of Violence

You open your mind to the violence of the violence of the material planes. When you make a melee or ranged weapon attack, you can use your abyssal weaponry feature regardless of whether or not you use sneak attack. When you do so, the tentacles deal necrotic damage equal to half the weapon's damage, including it's bonus damage dice for critical hits. 

Thoughts of Weaving

You open your mind to the strings of magic that connect the world like rivers. When you cast a cantrip that deals damage or use your sneak attack feature, you gain temporary hit points equal to the damage dealt.