Biomagnify

You will the cells of targets within range to stockpile negative energy. A target within range makes a Constitution saving throw or take 1d12 necrotic damage; creatures can willingly fail this saving throw. If a creature deals damage to your target while this spell is active, that creature takes 1d12 necrotic damage, and this spell ends. At 5th level, the aggressor creature takes an additional 1d12 necrotic damage (2d12). At 11th level, the aggressor creature takes another additional 1d12 necrotic damage (3d12), and you may target two creatures with this spell. At 17th, level the aggressor creature takes another additional 1d12 necrotic damage (4d12). Features such as some clerics' Potent Spellcasting only apply to damage dealt to the aggressor creature.