Sanguine Brotherhood

A Human bleeds out a blade before charging at her foes. A Half-Orc lashes out against the beasts attacking him with spikes made of his blood. A Kalashtar reabsorbs the blood they have bled out, ready to outlast the shadowy monstrosity they set out to destroy.

A hidden quasi-supernatural martial art order developed by descendants of those who survived Vorald the Dracula’s nightmarish reign, the Sanguine Brotherhood empowers warriors through a modified hemopathy technique, enabling them to fight with their blood and single-minded focus on bloodshed, from them or their foes. When a Sanguine Brother or Sister fights, it always gets messy.. But they’re efficient. And deadly.

Crimson Armory

Starting when you choose this martial archetype at 3rd level, you learn to shape your blood into tangible armaments. As a bonus action when you have at least one empty hand, you can take irreducible and unpreventable damage equal to half your fighter level (minimum 1) to create one of the following manifestations for 1 minute. The effect ends early if you dismiss it (no action required) or if the manifestation leaves your possession.

Blood Weapon. You create a melee weapon of your choice (simple or martial) out of your blood in an empty hand. The weapon is magical, deals normal damage for its type, and you can use your Constitution modifier instead of Strength or Dexterity for attack and damage rolls with it. In addition, you add half your proficiency bonus (rounded up) to damage rolls with this weapon. 

You can maintain only one Blood Weapon at a time.

Blood Shield. You form a shield of hardened blood around one of your arms. Once per turn, when you take damage from a creature you can see, you may use your Reaction to reduce the damage by 1d6 + your Constitution modifier. This reduction increases to 2d6 + Constitution modifier at 10th level, and 3d6 + Constitution modifier at 18th level.

You can maintain only one Blood Shield at a time.

Hemotic Infusion

Also at 3rd level, you learn to push your body past its limits for greater strikes. When you hit a creature with a Blood Weapon, you can choose to take damage equal to half your fighter level (minimum 1). If you do, the attack deals an additional 1d8 necrotic damage.

You can use this feature a number of times equal to your Constitution modifier (minimum of once). You regain all expended uses when you finish a long rest.

The bonus damage increases to 1d10 at 10th level, and to 1d12 at 18th level.

Hemorrhage Carapace

At 7th level, your mastery of blood hardens you against harm and punishes those who wound you. You gain the following additions to your Crimson Armory features:

Blood Weapon. When you take damage from a hostile creature within 5 feet of you, that creature takes necrotic damage equal to your Constitution modifier (minimum 1).

Blood Shield. While you are maintaining a Blood Shield, you gain a bonus to AC equal to half your proficiency bonus (rounded up).

In addition, when you take damage from any source, you can use your reaction to immediately create a Blood Weapon or Blood Shield without paying its usual hit point cost.

Reflexive Bloodlust

At 10th level, you have trained your body to turn the pain of fresh wounds into immediate retaliation. Whenever you take damage equal to or greater than half your fighter level from a single source, you may use your reaction to make one melee weapon attack with a Blood Weapon against a creature within range.

If this attack hits, it deals an additional 1d8 necrotic damage and that creature has a disadvantage on its next attack roll.

Crimson Vitality

At 15th level, you have learned to overclock your body, recycling your life force into renewed endurance. When you take damage equal to or greater than half your fighter level, you regain hit points equal to your Constitution modifier at the start of your next turn. Until the end of that turn, your movement speed increases by 10 feet as your body surges with adrenaline.

In addition, once per short or long rest, you can use your action to restore hit points equal to twice your fighter level + your Constitution modifier as your blood rapidly knits itself together.

Inhuman Heart

At 18th level, your heart has evolved to embody the pinnacle of the Brotherhood’s bloody art: a source of lifeblood made into a forge and fortress. Your body can now sustain more blood loss and strengthen your Sanguine techniques further. You gain the following features:

• You can maintain both a Blood Weapon and a Blood Shield simultaneously.
• When you roll initiative and don’t have any Blood Weapon, you may create one without paying its usual hit point cost. 
• Once per turn when you take damage, you can use your reaction to deal necrotic damage equal to your Constitution modifier + half the damage taken to one creature within 10 feet of you. You can use this feature a number of times equal to your Constitution modifier, regaining all expended uses when you finish a long rest.