Retiarius

Usually a fighter hailing from the high seas or costal regions, the Retiarius is a deadly warrior on land, and a terrifying force of destruction in the water.. Many of these fighters gain fame through battling in the pits as gladiators. These fighters get one up on their opponents in any aquatic situation. They also use their skills to gain mastery on entrapment and entanglement. Their preferred weapons are spears and tridents, which allow them to easily dispatch anything they have caught.

Upon choosing this fighting Archetype, you gain Neptune's mastery and Snare of the sea. With Neptune's Mastery, all strength rolls when dealing with tridents or spears turn into dexterity rolls. They are now more at home in the water then they were on land gaining bonuses while on ships or in the water. With the Snare of he sea one becomes better with entrapping creatures in a net, and does bonus damage to anything that has been trapped.

At seventh level fighter you are blessed by Neptune. You gain the ability to rejuvenate when in contact with water(In contact with water is in the DM's discretion if a water canteen would or something along those lines would work.)

When reaching a tenth level fighter you gain access to the spell Horrors of the deep. This summons a chuul to aide you battle.

After reaching a fifteenth level fighter you can choose one item from Neptune's own armory. Whether it be the offensive trident or the defensive breastplate, you will gain the items that will allow you to reach a new level of power.

The final gift from Neptune is received at an eighteenth level fighter. You gain the ability to summon a water elemental once a day to fight by your side and crush your opponents.

 

(This class is inspired by Roman Gladiators who went by the same name. With a few artistic liberties of course. This Archetype works well with DEX builds as it changes strength modifier to a dexterity modifier when using tridents or spears. I wanted to create a class that changed up the fighter slightly and though it is very niche, has a distinctive play style. I'd love feedback and anything you would think would help improve this class.)

Neptune's Mastery

These fighters gain bonuses to their attacks while they are wielding a trident(or spear like item). When using a trident use your dex modifier instead of when rolling for attack and damage.  These fighters are at home in the water and on ships and gain advantage on all DEX saving throws and DEX ability checks when in the water or on a ship. They are no longer slowed by water, have a swim speed of forty five, and can hold their breath twice as long as they could originally.

Snare of the Sea

You have advantage on all net attacks against humanoids or creatures smaller than you in size. Creatures also have disadvantage on trying to escape your nets once trapped. You force a DEX saving throw at a 15 Dc to escape your net. You also deal 1d6 bonus damage to restrained targets, which gains in power as you level up.

At level 5 the damage is increased to 2d6 and at level 10 it is increased to 3d6. At level 15 the dc increases to 18.

Blessing of Neptune

At level 7 you gain access to a pool of healing that you can use to heal yourself or allies that are in contact with water(you must be in water to use it). After a long rest a Retiarius rolls a number of d4s equal to their fighter level. 

(Similar to Paladin's lay on hands, except you have to roll for your pool of healing, it only takes a bonus action, can only heal yourself, and have to be in contact with 5 sqft of water. This does include streams and rivers. Any other imaginative options are up to the DM's discretion.)

Horrors of the Deep

Once per day you may cast Horrors of the deep, which summons a chuul to fight for you. When summoned it has its own inititave but is controlled by it's summoner. It lasts for an hour unless it is dismissed or killed.

Armory of the sea

You are able to choose between Leviathan's Hide or Neptune's Trident.

Leviathan's Hide

https://www.dndbeyond.com/magic-items/1223529-leviathans-hide

Neptune's Trident

https://www.dndbeyond.com/magic-items/1223507-neptunes-trident

Leviathan's Hide

While your health is above 50% your Ac is set to 15, your DEX Score is set to 20, and you deal an extra 2d4 force damage to every melee attack that requires Dex. Once your health drops below 50% your ac is set to 18, you gain resistance to all non magical physical attacks, and get advantage on all saving throws.

Neptune's Trident

A blue trident tipped with gleaming golden spear heads. The trident does a base 2d6 piercing damage and does 2d6 cold damage. You can use a bonus action to change the cold to force or thunder damage. After every short rest you get three charges of Electrical Conduit. You call down a strike of electricity onto an opponent. They must make a 15 Dc DEX saving throw. On a success they take half damage and have advantage on the subsequent con save, and a failure they take 3d10 thunder damage. Along with damage they then must make a 15 Dc Con save to avoid paralysis.

Boon of Neptune

You gain the ability to summon a water elemental once per day. It disappears when it dies, if the caster dies, falls unconscious, or dispels it. There is no time limit on how long this creature can stay under the command of the caster.