Ebb and Flow

You tap into the life force of a 15-foot-radius sphere centered on a point within range. Each creature of your choice in that area must make a Constitution saving throw, taking 4d10 necrotic damage on a failed save, or half as much damage on a successful one. Nonmagical vegetation in that area withers, and any food or water becomes rotten.
Any creature in the area you chose to exclude from the damage may instead heal. The creature may spend and roll its unspent hit dice and regain a number of hit points equal to the roll plus your spellcasting ability modifier. The creatures may roll up to four Hit Dice and applies your spellcasting ability modifier to each roll. All Hit Dice must be rolled at once, the creature may not choose to roll additional Hit Dice separately. 
At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the radius increases by 5. Additionally, the damage increases by 1d10 for each slot above 3rd, and the number of Hit Dice that can be spent and added to the healing roll increases by one for each slot above 3rd.