Your innate magic comes from your life force and the blood in your veins.  Technically speaking, all blood has magical qualities, but those who are able to tap its potential are few and far between.  Every mortal being is born with a certain potency of magic in their veins.  Most have almost no magic, and either gain magical abilities through other means or live to lead mundane lives.  Those born with potent magical blood usually find that blood toxic to them, overwhelming and killing them in infancy.  Only one in a million has blood that is significantly powerful without being fatal.

Unlike other magical bloodlines, the arcane blood is not linked to a powerful ancestor or a gift from an extraplanar being.  It has never even been found to be heritable.  The arcane blood is a true anomaly.  Most people have never even heard of it.  Many of those who have see the act of drawing blood to cast magic as something twisted and perverse, often mistaking the practice as a fiendish ritual.

Some beings see this power as a gift, granting them potent magical abilities.  Others view it as a curse, subjecting them to side glances and ridicule.  Regardless of your opinion, your unique blood grants you abilities unavailable to any other mage.

Blood Magic

Beginning at 1st level, you can tap your blood to gain a magical advantage, using special dice called blood dice.  You have a number of blood dice equal to your constitution modifier.  When you expend a blood die, you extract blood from your body, taking necrotic damage equal to the number rolled.  This damage ignores temporary hit points, resistance, immunity, and vulnerability.  At 1st level, your blood die is a d4.  It becomes a d6 at 5th level, a d8 at 9th level, a d10 at 13th level, and a d12 at 17th level.  All expended blood dice are regained after a long rest.

Hematic Affinity

Beginning at 1st level, your life force can be infused into your spellcasting.  Once per turn, you may expend a blood die to add your Constitution modifier to a spell attack roll for a spell cast that turn.  Alternatively, you may expend a blood die to add your Constitution modifier to your spell save DC for a single spell you cast that turn.

If a spell requires you to make multiple attack rolls, you may apply your Constitution modifier to all attack rolls required by this spell.

This feature cannot be used on the same turn as Bloodied Fury.

Bloodied Fury

Beginning at 1st level, your blood can be drawn to amplify your harmful magics.  Once per turn, when you deal damage with a spell, you may expend a blood die and add the roll to the damage for the spell.  The damage dealt with this feature is of the same type as the damage dealt by the amplified spell.

If the affected spell produces multiple projectiles (as with magic missile), the additional damage caused by this feature is applied to only one projectile. An additional blood die may be expended only for a projectile that strikes an additional target.

This feature cannot be used on the same turn as Hematic Affinity.

Sanguine Restoration

At 6th level, you can tap your own life force to feed your font of magic.  As a bonus action, you may expend any number of blood dice and regain sorcery points equal to the number of blood dice expended.

Crimson Gift

At 14th level, you're able to extend your innate magic to others.  As an action, you may expend a blood die to grant a willing creature within 5 feet temporary hit points equal to the number rolled on the die.

Dark Transfusion

At 18th level, your familiarity with blood magic grants you limited control over the life force of others.  When a creature within 5 feet of you takes damage, you may use your reaction to expend a blood die.  The target takes necrotic damage equal to the number rolled, and you regain an equal number of hit points.  The blood die expended by this feature does not cause you to take damage.  Once you use this feature, it cannot be used again until you finish a short or long rest.