Touch of Hadar

Your hands or fingers become twisting, purple tentacles. When you cast this spell, you make a melee spell attack against one creature within 5 feet of you. On a hit, the target takes 1d10 necrotic damage. Hit or miss, until the start of your next turn when a hostile creature provokes an opportunity attack from you, you can use your reaction to make a spell attack with this spell, rather than making a weapon attack. Your limbs or digits return to normal at the start of your next turn.

At Higher Levels. The spell’s damage increases by 1d10 when you reach 5th level (2d10), 11th level (3d10), and 17th level (4d10).