In order for this spell to be effective the target must have taken at least 1 point of slashing, piercing, bludgeoning, or force damage before the spell is cast. 

The caster carves the rune of the ouroboros into their chest. They sacrifice 1d4 HP. 

Upon casting, the target's wound erupts into the shape of the rune carved onto the user and the blood curse is placed. The initial curse's eruption deals 2d8 necrotic damage.

Once the curse is placed, a misty trail of blood drifts in the air between caster and target. Each turn that this curse is maintained, the caster may deal and additional 2d8. This spell can be broken by the use of Remove Curse.

At higher levels the damage delt and received increases:

3rd level > offer 1d6 and deal 3d8

4th level > offer 2d4 and deal 4d8

5th level > offer 1d8 and deal 5d8

6th level > offer 2d6 and deal 5d10

7th level > offer 3d6 and deal 6d10

8th level > offer 2d10 and deal 6d12

9th level > offer 12 hit points and deal max damage of 7d12