Before you cast this spell, you must expend a number of hit points equal to the result of 1d6, or the spell automatically fails. Your target does the same, but only takes half of the result instead of the full 1d6 dice roll.   You cut yourself and one other willing creature, mixing your blood. Once before the spell ends, the target can subtract the number of hit points expended in total to cast this spell from any nonmagical damage they take. When they do this, this spell ends.   When you cast this spell, you can choose to have the number of hit points you expend increase to 1d8 at level 3, 2d8 at level 5, 4d8 at level 7, and 6d8 at level 9.