Base Class: Sorcerer
While some sorcerers are bursting at the seems with magic, blood casters are not so lucky. A blood caster does have magic flowing through them, but the need to practice and train to use it. Blood casters can cast spells at a normal ability, but if they need to, they can use their own blood to make their spells stronger or twist them in strange ways. A blood caster might draw their blood to up cast or use meta magic. Blood casters gain access to special abilities through blood rituals and will sacrifice long and short term health for moment to moment advantages in combat.
Blood price
Starting at first level, you can use health points to upcast your spells. To upcast a spell, it costs your sorcerer level plus the level of the spell slot you want times your constitution modifier.
Twist fate
Using the magic potential in your blood you have the ability to bend fate by adding one d4 to an ability check even if the check. This can be used once per short rest and costs one health point.
Curse of the frail
As your attack make a telepathic link with an enemy of your choice, they must succeed a intelligence save of you spell save DC, if they fail, they receive vulnerability to a damage type decided by you, however, all damage done to them of any type other than the one you chose, is halved and reflected onto you. At level 15, you get to choose an additional damage type, but only if you want to.
Transfusion
With your ever growing knowledge of blood, you can temporarily control how it flows. Once every long rest, you can use your action, you can cast transfusion, a spell that allows you to heal people with the life force of others. For the next 3 turns, every time you damage an enemy, you can use a bonus action heal one party member within 40ft or yourself by 3d10.
Blood lust
Spend your bonus action to enter a state called blood lust for the next ten turns, when in blood lust, all spell attacks are rolled with advantage and you deal an additional 25 slashing damage to all attacks, the rage can be ended whenever you want with no cost of an action or bonus action, however, every turn you remain in blood lust, you take 20 damage yourself.