Order of the Sunless

Basically vampires.

Vascular Enhancement

Starting at 3rd level, once per turn, whenever you make an attack roll, damage roll, ability check or saving throw that uses your Strength or Dexterity modifier, you can choose to roll one hemocraft die. You add the result to your initial roll by losing an equivalent number of hit points. You can do so after seeing the initial roll but before the DM announces the outcome of the roll. 

You can only use this feature if you have more than half of your hit points left.

Supernatural Transfusion

Starting at 3rd level, when your hit points fall below half your hit point maximum, you gain a natural weapon as sharp fangs extend from your teeth. Your fangs count as a simple melee weapon for you, and you add your Constitution modifier to your attack and damage rolls with it. 

On a hit, it deals piercing damage equal to one roll of your hemocraft die, and you regain hit points equal to half the damage dealt. You can attack with your fangs as a bonus action after taking the Attack action, but you can only attack with your fangs once per turn. 

Child of the Night

 Beginning when you reach 7th level, you can use a bonus action to transform into a swarm of bats. While you are in this form, you retain your Intelligence, Wisdom, and Charisma scores, and you can’t take any actions other than to Dash, Disengage, Hide or Search. You revert to your normal form at the end of your turn or as a free action during your turn.

You assume the hit points of your new form. When you revert to normal form, you return to the number of hit points you had before you transformed. If you revert as a result of dropping to 0 hit points, any excess damage carries over to your normal form. As long as the excess damage doesn’t reduce your normal form to 0 hit points, you aren’t knocked unconscious.

You can use this feature a number of times equal to your Intelligence modifier, and you regain all expended uses when you finish a short or long rest.

Brand of Siphoning

Starting at 11th level, you are always under the effects of a spider climb spell. Additionally, your Brand of Castigation lets you drain the vitality of your prey. Once per turn, the first time you hit a branded creature with an attack, you regain a number of hit points equal to one roll of your hemocraft die + your Constitution modifier.

Blood Curse of the Thrall

At 15th level, you have learnt to use your hemocraft to temporarily control your enemies. You gain the Blood Curse of the Thrall for your Blood Maledict feature. This doesn’t count against your number of blood curses known.

Blood Curse of the Thrall

Prerequisite: 15th level, Order of the Sunless

As a bonus action, you telepathically issue a command to a creature within 30 feet of you. The creature must make a Wisdom saving throw or become charmed until the end of its next turn. While the creature is charmed, it does its best to obey your command. You can specify a simple and general course of action, such as "Attack that creature,” "Run over there,” or “Fetch that object."

Amplify. This curse lasts for 1 minute, and you can telepathically issue new commands to the creature as a bonus action on your turn. If the creature completes the order and doesn’t receive further direction from you, it defends and preserves itself to the best of its ability. The creature may repeat the Wisdom saving throw at the end of each of its turns to end this effect.

Ichor Mastery

At 18th level, you can use your Vascular Enhancement and Supernatural Transfusion features regardless of your current hit points.