Viola's Blood Pulse

You are able to drain the very blood from the veins of your enemies amplifying the suffering of wounded creatures. Each creature of your choice in a 20 ft. radius sphere must make a Constitution saving throw. On a failed saving throw, a creature takes 4d8 necrotic damage and cannot regain hit points until the end of its next turn on a failed saving throw. On a successful saving throw, creatures take half damage and are otherwise unaffected. 

If a creature is missing any of its hit points, it instead takes 4d12 necrotic damage.

At Higher Levels When you cast this spell using a spell slot of 5th level or higher, the extra damage increases by one die respectively.