Circle of Scarlet (Sangromancy)

As part of casting this spell, you must expend four hit dice or the spell automatically fails. If you do, choose a point in the ground within range and roll the expended hit dice. A crimson pillar erupts from the ground in a 15-foot high cylinder centered at the point you chose. Each creature within the cylinder must make a Constitution saving throw. On a success, the creature takes necrotic damage equal to the total of the expended hit dice. On a failure, the creature takes necrotic damage equal to the total of the expended hit dice and rolls four hit dice of its own, taking additional necrotic damage equal to the result. 

For each creature that fails the saving throw against this spell, you gain 5 temporary hit points.