Tides of Blood

You gather a large ball of blood and magic in your palms. After reaching full size you hold it over you head and it explodes outward in a 15 foot sphere around you. Creatures in the area must make a Dexterity Save or take 3d6 Necrotic damage and the next attack on them will trigger this damage again (the same damage you dealt with the initial cast). On a successful save the target only takes the initial 3d6 Necrotic damage.