Barrier's Training.

Warriors who draw power from the tropical waters, Barriers harness the primal magic of the reef to don a spiked armor made of living creatures. Because of their connection to the natural world and potent martial prowess, Barriers are commonly fielded as champions and defenders among coastal cultures for their unbreakable endurance and ability to defend their companions.

Level 3 Features:

Mantle of Coral.

Upon taking this archetype at 3rd level, you can use a bonus action on your turn to create a suit of magical living armor around you, which lasts for 1 minute. You can create this armor on top of any existing armor you are wearing. While you have this armor, you gain the following benefits:
-Your AC can't be less than 16.
-If a creature within 5 feet of you hits you with a melee attack, the creature takes 1d4 piercing damage. This damage counts as magical for the purpose of overcoming resistance and immunity to nonmagical attacks and damage. The first time a creature takes this damage on a turn, its speed is reduced by 10 feet until the end of its next turn.
You can use this feature twice, and regain all expended uses of it when you finish a short or long rest.

Expel Toxin.

Also at 3rd level, you take on the natural healing properties of coral, and can use it to expel yourself of poisons. When you regain hit points using your Second Wind feature while your Mantle of Coral is active, your armor deals an additional 1d6 poison damage until the start of your next turn, and you become cured of one disease or poison affecting you.

Level 7 Feature:

Fed by the Sun.

At 7th level you can draw upon the light of the sun to nourish and sustain yourself. If you spend 1 hour in bright sunlight, you regain hit points equal to your Constitution modifier (minimum of 1 hit point), and gain enough nourishment to sustain yourself for one day.

Level 10 Feature:

Bristling Defence.

Starting at 10th level, when a creature within 5 feet of you that you can see hits you with a melee attack while your Mantle of Coral is active, you can use your reaction to conjure bristling spines from your armor. You halve the attack's damage against you, and the attacker takes additional damage from your armor equal to your fighter level.

Level 15 Feature:

Frenzied Swarm.

Beginning at 15th level, you can manifest creatures of the reef from your living armor to aid you in battle. As an action while your Mantle of Coral is active, you can conjure a swarm of quippers, which appears in your space.
When conjured in this way, the swarm can breathe air and water, has a flying speed equal to its swimming speed, and appears with temporary hit points equal to twice your fighter level. Additionally, the swarm adds your Constitution modifier to attack and damage rolls it makes, and its attacks count as magical for the purpose of overcoming resistance and immunity to nonmagical attacks and damage. When the swarm deals damage to a creature, the creature's movement speed is reduced by 10 feet until the end of the creature's next turn.
The swarm obeys your verbal commands (no action required by you), and shares your initiative in combat, but takes its turn immediately after yours. The swarm disappears when it drops to 0 hit points or when your Mantle of Coral ends.
Once you use this feature, you can't use it again until you finish a long rest.

Level 18 Feature:

Barrier Reef.

At 18th level, your Mantle of Coral increases in potency and protective power. When you activate your Mantle of Coral, you gain temporary hit points equal to your fighter level, which last until your Mantle of Coral ends.
Additionally, the piercing damage from your armor increases to 2d4, and a creature that takes this damage has its movement speed reduced to 0 feet until the end of its next turn.